Wait, who's the third dude on the left?

The dude on the left is Krissy. Early into writing a springboard "Pilot" chapter for UBM_Concept Toby needed someone to vent his problems to to set up the conflict of "I am going to quit my job because my human kid is a mid-20's adult woman now and is trying to seduce me but I am also interested in her and I shouldn't be" so a coworker from a different department (A bigfoot in the woods monster instead of Under the Bed Monster) was made and thus Krissy was born. She ended being added to the plot as a comic relief character who acts like a big Himbo, loves stealing human stuff from the campsites she torments, gives painfully simple but effective advice that the over complicating Toby never takes, has four siblings that look exactly like her, and is in a long term relationship with a trained assassin she can't break up with because they're basically a ninja and she can't FIND them outside of the occasional laser target sight aimed at her forehead.

So Toby needlessly over-thinks everything instead of just
A. Asking to be reassigned
B. Firmly telling Margot no (instead of "No, please don't seduce me with your human wiles and don't wear that dress I keep mentioning looks good on you. Someone save me."
C. Just giving up and doing the interspecies tango.

That's basically Toby in a nutshell.
No matter how simply you slice it he will always insist it is far more complicated than it really has to be!

Also, question: Why did you make Krissy the most fun character ever?

Because Krissy is simultaneously the comic relief but also the realest character so thus has the perfect character trait combination for mass appeal! While also being a great counterbalance to both Toby and Margot's drama,
as Margot has this "Don't give a shit, love wins" personality without considering that there IS complications to the relationship she wants to start with Toby, from species difference, to the whole "Monster's Inc." style different worlds, down to the fact that Toby probably can and WILL get in trouble for not only not reporting that his assigned child is very much no longer a child and no longer afraid of him, but now he's potentially FUCKING her, Margot very much doesn't consider the consequences that they have potentially ahead of them.
Toby on the other hand CANNOT stop over complicating and overthinking any given scenario. If there is one GOOD outcome there is a thousand potential ways the whole world will end instead, and yet he won't just walk away from it either which would technically solve his problem but as much as he won't admit it he ALSO likes Margot and doesn't want to break it off or be put into a situation where he no longer sees her.

Then there's Krissy who's perfectly in the middle, bringing in a level of Himbo comedy with the sensible audience pov of
Toby: "I can't do this anymore!"
Krissy: "Then don't."
Toby: "But I don't know what I would do then!"
Krissy: "Then do."
Toby: "BUT THEN EVERYTHING WILL CATCH FIRE!!!!!"
Krissy: "Then... Don't?"
Toby: "BUT THEN---"
Krissy: >:I ".... Go eat lunch with your girlfriend. I bail you out in 15 minutes so you don't get frisky."

I'm not trying to put the blame on Toby, but if Margot has known him for so long, I'm not surprised that she's dismissing any real concerns as just him being worried about nothing or exaggerating the problems.

I mean for Margot the reality of the whole situation doesn't fully exist? Toby has been her assigned Monster since he was an intern and she was a child and the worst thing that's ever happened to her since knowing him is... initially being afraid of him. Other than that it's just been him showing up in her room regularly and her harassing him and him coming back for more and nobodies caught them yet and there have been no consequences yet so it looks from all accounts like Toby is just being a worry wart, which tracks with his personality