On behalf of General Custer, let me welcome you to the last stand.

Animorphs have this really important rule: never morph a sentient being's without their permission. IC-ly wise, Marco will usually respect this rule unless he doesn't. When he feels he will need to get the upper hand in any situation, he will take the DNA and morph into that particular person, permission or not.

That's where you come in. See, the neat thing about morphing is that it is purely DNA-based. For example, you have a character that is extremely scarred from war wounds. Marco acquires that person's DNA and morphs into that person, but without the scars as the scars are not something the character was born with. Basically: everything your character is born with, and if completely part of his/her DNA, then Marco will have it. Genetic memories, powers, everything.

So tell me if Marco can morph into your character and what he may or may not have while doing so! If you don't want him to turn into your character, we can find ways that Marco won't be ever having the need to do so.