Frankly speaking, I do not think this is among the best of cards.

• The risk. Drawing just one, two, or no cards if you draw a bad token.
• The actions. An action to play and more actions to use. The net gain is very fast on good token draws, but you're still depending on luck! I find 3 draws at 5 a rather stable result on standard.

There's also a resource cost but as a you don't care about that much.

So, when is it good?

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Not for every Daisy Walker, she's busy with her Old Book of Lore, the arcane slot also has lots more competition. It might be worth a shot once the book turns into Encyclopedia, but that's often very far down the line.

Not for Norman Withers, obviously.

Rex Murphy can put it to use, you can definitely build a deck with it to quickly draw all the tools and support to keep you moving and clue-ing. But be aware that you'll see a lot more of his weakness.

Ursula Downs can use it too, but again in a fairly "alternative" kind of way, just to draw her deck faster and find the movement assets, the plan more consistently.

And finally, in my opinion where this card is at its finest, Minh Thi Phan. Minh likes skill cards, she likes Cornered to work around the weakness and to use with Strange Solution, Cornered + Strange Solution means that she fights at +8, very nice for a , and Feed the Mind fuels it with cards to discard.

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So, at the moment, a utility card that's just got one proper deck where it really really shines. Try it. I recommend it.

This card is underrated. Imagine a fast 1-cost guardian card that gives +2 combat and +2 agility until the end of the investigator's turn. That would be an auto-include in many combat-focused decks. So why is this card so little valued when it basically does that (at least against one enemy)?

I’m having a bit of a hard time quantifying the strength of this effect. It seems like it has about a 50% chance of combining a bad token draw with a good one, thus giving you a great choice. Two good tokens or two bad ones drawn together is not as helpful. Also it seems to be better on higher difficulties, where the difference between good and bad tokens is larger. But I think the card might be a bit overrated because of the high XP and the significant chance of drawing two similar tokens.

Four reviews for Joe and none for Carolyn, the most interesting investigator in the game? This will not stand.

The Combat Medic

Carolyn Fern is an especially durable Guardian with a powerful ability and effect. Additionally, her deckbuilding options are extremely broad, enabling great flexibility in her builds. This is both a blessing and a curse (appropriately), as one can feel inundated with decisions and suffer from option paralysis. Additionally, the devs seem to value flexibility very highly when creating characters, resulting in Carolyn having a base stat total of only 11. Her increased durability (she has an extra point in health/sanity) offsets this, but I would say a stat point is more valuable than health/sanity, so it's a bit meh.

What Makes Her Great

Carolyn's deckbuilding options combined with her ability open up a massive array of options that totally warp how a player should value cards. What do I mean by this exactly? Take First Aid, a fairly lackluster card that few Guardians can find room for. In Carolyn's hands, it can not only bring an ally from the brink of insanity but also allow them to afford their expensive assets. Clarity of Mind becomes a similar tool for both saving and paying allies, despite being maybe the worst horror heal in the game overall. Once higher level cards become available the the player, one's options expand into truly ridiculous healing tools like First Aid(3) and Ancient Stone(Minds in Harmony). On the other hand, those two items are basically the only high-level horror heals that your allies will care about.

A sidebar on Ancient Stone: With Shrewd Analysis, one can upgrade two copies into Ancient Stone (Minds in Harmony) for only 3xp. This is well worth one / slot, especially since the stones free up their slots after being upgraded. Identifying it can be tricky without devotion to clue-gathering, but shooting for a total of 4+ is very strong, since it neatly counters Rational Thought.

Basically, Carolyn fundamentally alters the value of many sub-par cards, increasing the amount of viable includes while deckbuilding.

Core Builds

Carolyn has so many options that it can be helpful to focus on a strategy and include some core elements, filling out the rest of the deck as one sees fit. To that end, here are some effective cores:

Cluever

Dr. Milan Christopher
Alyssa Graham
Magnifying Glass
St. Hubert's Key
Deduction
This is a build with very strong static increases to investigation. With everything in play (which ostensibly includes Charisma, a superstar include for Carolyn) the investigator is up to 8 while investigating. This would require a massive amount of resources, but I have done it. It also makes identifying the stones fairly easy.

Support

First Aid
Clarity of Mind
Liquid Courage
Inspiring Presence
With these assets Carolyn is able to bankroll her allies, feeding money to the fighter so they can afford their expensive weapons. With xp, Stand Together is an incredible value both xp and card-wise. Inspiring presence can also be used to heal and recharge your own or your allies' Beat Cops.

Combat

Guard Dog
Beat Cop
Shrivelling
"I've got a plan!"
Dynamite Blast
This build uses guaranteed damage to deal with enemies and Carolyn's tankiness and healing to soak shots. Between her ability, Stick to the Plan and Emergency Cache, she has consistent access to guaranteed burst damage, despite it being fairly resource/xp intensive. In any case, "I've got a plan!" is and has always been a very strong outlet for damage, despite its one-shot nature, and I feel Shriveling would need a lot of support (from St. Hubert's Key/Holy Rosary, Arcane Studies, and/or Peter Sylvestre(2)) to be effective late-campaign.

Other Strong Cards for Carolyn

Peter Sylvestre
The big man on campus can provide you with a constant stream of resources, while providing his static stat boosts and infinite horror soak as usual. Combined with a reliable way to take horror, Peter becomes an enormous economy card rivaling Dr. Milan Christopher. Speaking of which...
Forbidden Knowledge
With Carolyn's ability to heal horror, this is very safe to take. If Peter Sylvestre is in play, you double your resource gain and take no horror, which is quite strong. Cautious players (or those on higher difficulties) might opt for Painkillers instead, since it can provide fast, reliable horror in much the same way while shoring up Carolyn's low health.
Charisma
In every Carolyn deck I've ever built, I've ended up with more than four ally cards. Especially if you decide to go with Foolishness, one or even two copies of Charisma will enable Carolyn to maintain powerful effects on the board, such as Dr. Milan Christopher and Peter Sylvestre, or Alyssa Graham and Dr. Henry Armitage, or Foolishness and Peter Sylvestre. Her allies are incredibly useful.
Ever Vigilant
Similar to Charisma, Ever Vigilant allows Carolyn to setup for the whole game in a single action. I prefer assets over events for horror heal, so every deck I build with Carolyn has more than 20 assets. In this case, Ever Vigilant is incredibly strong and will likely be useful even late in the game, especially if you've found yourself without actions to play cards and great assets building up in your hand. This can also go under Stick to the Plan, although Carolyn doesn't have quite as many viable options for that card, so it might be overkill. I think taking aggressive mulligans for Ever Vigilant is probably the better strat, as it saves you 6xp.

The most effective strategy is probably to combine two of these cores into a deck that can take useful actions at any point in the game, for example by combining the Cluever with the Support suite. Or one can be more adventurous and combine everything they think is cool into some kind of monster. The options are quite vast.

Signature Cards and Weaknesses

Hypnotic Therapy is a very good card. Being able to heal one horror from any investigator every turn for the rest of the game is ridiculous, and it draws that investigator a card, and you can give them a dollar with your ability. If this is the only horror heal you get in play, things start to stabilize instantly. If you get it into play later, it's a steroid for your other horror heals, so that's great too. I wish it didn't exhaust for both effects, but it would definitely be broken if that were the case. Rational Thought, her corresponding weakness, isn't nearly as bad as To Fight the Black Wind, especially with Hypnotic Therapy or Ancient Stone (Minds in Harmony) with four secrets on it in play. It is still very annoying, since you don't get the benefit of your ability while it's out, so it can hurt your income. In my experience, it hasn't affected me at all, but I have yet to draw it when we're all about to go insane and I'm low on healing options. In any case, Alyssa Graham is useful to avoid such occasions.

Foolishness, her replacement signature, is very good, but also very annoying. Aside from needing a bunch of support to come online (which, admittedly, pays you back a bit), he also takes the ally slot, which is so incredibly valuable. I think he becomes more attractive in a four-person party where other investigators need Dr. Milan Christopher or Peter Sylvestre more than you do. To Fight the Black Wind, the corresponding weakness, is extremely terrible and can result in the agenda flipping a turn early if drawn in the upkeep phase. Ancient Stone (Minds in Harmony) is also very good against it, as long as you draw it with some actions to spare. There's certainly an argument to be made for taking all of her signature cards, since they do have some synergy with each other, but it is resource intensive.

Conclusion

Carolyn is one of my favorite investigators, probably for the same reasons that many seem uninterested in her: she's super weird. Her versatility is incredible, her ability is interesting and strong, and has no phase/turn limit, and her stats are strong enough that she's a competent investigator before applying any bonuses. Still, she has her downfalls: I've frequently found myself in scenarios with no horror to heal, which makes me value Forbidden Knowledge a little more highly. Her effectiveness is also highly campaign-dependent, I feel; she has plenty of horror to heal in NotZ and Carcosa, but less to do in Dunwich. In Forgotten Age, I think she's downright bad because the campaign throws far more damage at one than horror and her is garbage. With its focus on "the ethereal," I'm going to assume the Circle is also heavy on horror, so she might function quite well there (investigators are generally pretty powerful in their release campaigns).

Anyway, I hope you've enjoyed this review of Carolyn Fern. Thanks for reading.

Edits:
Carolyn can't take Monster Slayer; I'm an idiot. Good catch @Death by Chocolate.
Added Dynamite Blast in the Combat build; she's one of the few s who can afford it, and I forget how good it is; good call @Swekyde.

I'm in disagreement with everyone that says this is a horrible weakness. Whenever I draw this card I actually feel lucky I didn't get something worse. To understand why, just imagine this scenario:

You are Daisy and you are engaged with a powerful monster, you can kill it with your trusty shriveling and higher education, but you only have 1 action left and you cannot afford to take another hit. You decide to evade the monster so you can kill it the very next round. You succeed, but you had to commit more cards than you wished to. Now you have four cards left. No big deal, or so you think, after all the upkeep phase is coming and you'll get your fifth card. You draw a card and it is a basic weakness...

Now the next round begins, having a weakness in this situation is already bad as it is, but there is another very bad effect of having drawn a weakness. Something that whoever thinks "Indebted" is awful is probably missing. You were robbed of the chance of getting a useful card. Now you have no way to use higher education to boost your willpower, you will not be able effectively attack the monster successfully. You will be defeated and you will lose the scenario.

A basic weakness in your deck doesn't simply do whatever it is written on its revelation effect, it also robs you of the chance of getting a useful card. You spent an action to draw a card, but you got a weakness? Your action was wasted. You just used overpower and perception, but when you draw a card you get a weakness? What was meant to be a reward became a punishment.

It is true, unlike other weaknesses indebted will always hurt you. But there are two things that indebted will never do. It will never haunt you at a most critical time, and it will never rob you of a chance of getting a useful card.

If you were given the choice to get a consistent mild evil or something that inconsistently swings between neutral to devastating, what would you choose? If you choose the latter I can only say I envy you for feeling that lucky, I sure don't.