Okay, so as someone who's acquired the morphing technology, Tobias can morph into any creature he touches. This includes characters! So obviously I need a little input from you guys.
He can only copy instinctual information, so if your character has powers they were born with but couldn't be used without training he can't use those. If your character doesn't have DNA, he also can't morph them. Additionally he'd have to acquire them while in his red-tailed hawk form. ('Acquiring' just makes your character sleepy for a moment, and it's not likely that most will really notice anything wrong.)
In general the Animorphs have agreed not to acquire anything/anyone sentient who doesn't give permission, but they've been known to break this rule when they need to.
In addition! Tobias is from the '90s. '90s culture is pretty rampant through the books. Doom, Pokemon, Baywatch, Arnold Schwarzenegger, and other things are referenced, sometimes often. If your character's canon was around in the '90s and there's a reasonable chance that a dorky kid in California could have seen it, please let me know!
Can Tobias acquire your character?: If 'no' then ignore the rest of this part! Or if you're not sure, just ask and we can figure it out!
What species is your character/does your character consider themselves?: Particularly important if your character differs from 'human' at all.
Does your character have any kind of natural abilities/physical features?: This includes strong instincts characteristic of your character's species.
Does your character have scars or any sort of external, permanent injury?: Since morphing is from DNA, things like scars or missing limbs will regrow when he morphs. However, if your character has a condition like being blind or paralyzed at birth, then this won't fix it.
Is Tobias likely to know your canon?: Basically was your canon mainstream in America in the '90s? If not, you can ignore the rest of this part!
Did this canon exist in the form you play from in the '90s?: For example - he wouldn't recognize someone like Gandalf based on his face - but if he heard the name he might.
Can he fourth wall your character?: He's not outright likely to be like "hey you're from that show!" but he might react a little. Saying 'no' is okay!
Questions/Comments?: Free space! Anything you don't understand can go here.
Acquired list: Ostrich horse, Paarthurnax, anklegator.