Note that this card is doubly good for Mary. In addition to addressing her weak health, it can be used to get lots of extra [bless] tokens from Guardian Angel by assigning a damage to it and then canceling the damage.

Field agent might seem like the cluevery equivalent of Beat Cop, with one fewer uses, but indeed, she's much better. Forget the boost. It's nice, but secondary. If you build around this ally, you can turn her into possibly the mightiest no-test, no-action clue-gobbler in the whole game (barring the prohibitively expensive Agency Backup.)

That said, she does need some building around. There are a good handful of ways to heal/remove horror from allies, but I'll just mention three, since these do not require actions. With them alone, you can count on 5-6 free clues,and upward of 10-15 in the best-case scenarios, all while never pulling a token, and never spending an action (except to play her).

Trusted: if you have the Field Agent, think of this card as a discounted Working a Hunch, that also heals a point of damage. That's basically how it works out! Trusted can also be used to protect the field agent from forced damage/horror from treacheries (deal one direct horror to each card you control, etc.). That is a concern in some scenarios, since without Trusted she'll often be one horror from death in the Mythos phase.

Inspiring Presence: in the best case scenario, you're using Field Agent almost every turn. This card let's you use her twice in a turn, while only inflicting one horror on her.

Solemn Vow: The other two cards are handy, but this is what takes the Field Agent from good to godlike. Healing horror from investigators is usually much easier than from allies. Solemn vow let's you tap the field agent every single turn by instantly offloading the resulting horror onto an investigator. Of course, not all allies can absorb a per-turn horror, at least not without devoting significant resources to healing. But you know who can? Parallel Daisy. It's routine for her to have a sanity of 13 by the midgame, and not unheard of for her to reach 15 or even 16, way more than you'll ever need. Plus she usually has an ally who can soak a horror or two as well. Since Solemn Vow is fast to play and activated with a fast action, this horror conversion from Field Agent to Daisy is entirely actionless.

If you pull the Field Agent early, and your pal (ideally Daisy or Caroline) pulls a copy of Solemn Vow, your team will be in very good shape. Are you a one-intellect oaf usually engaged with two enemies? Don't let that limit you! With Field Agent, you STILL might be the leading clue gatherer for the scenario.

I like, that not only she can kick ass with her martial and melee fighting arts, but also she can throw Mk 1 Grenades here and there :D Too bad there are no other throwing weapons, like shurikens or daggers, other than Knife, that would maybe serve similar purpose like Wither

Subject 5U-21 can devour this card due to her Ravenous Forced effect or her own free-triggered ability, right? It is not a story asset, it is no longer in hand once revelation effect is carried out, so it can be choosen as a valid target for a non-story asset controlled by an investigator in her location, and therefore this weakness is a blessing in disguise for her. She can even chose the order of "at the end of your turn" resolution so not a single point of horror will be assigned to her. Or am I missing anything?

Since it is possible to devour an enemy or a treachery card (Reality Acid can let it happen via an elder sign effect), when you return this type of card (and I believe you can choose it), is it going to an encounter deck discard pile (or an investigator's discard pile if it is a weakness)?