I just started reading it yesterday so I don't have all the spoliers but I do have spoilers of the "uke's brother".
But before that the ML isn't a red flag at all he takes care of the uke in his puppy form. He finds out right away that the puppy he's taking care of is actually the beastman uke that worked at the casino but doesn't care. He still spoiles and teases him and the uke doesn't know that ML knows so he's trying to find a way to escape when his injuries heal and he can turn back human.
So the "brother" is an orphan that's part of an organization and runs a casino. He is racked up some depth some years ago so in order to pay it off he uses/steals from uke's paycheck to pay it off without him knowing. He "claims" that he found the uke in his puppy form abandoned 5 years ago and took him in. But it is most likely that he took some part in him being "abandoned" because he knows what type of beastman his parents were and that he is small most likely because he took after his toy sized maltese mom. He doesn't feel bad Stealing money from the paycheck because "I deserve it since I took care of him for 5 years"
He sends the uke to ML to do one get one last drug package or whatever because he only trusts him, then promises to the uke that after this they will have a normal life. BUT hes actually plans to frame/kill the uke so that he could run away with the drug money, paying off his remaining depth and settle down to marry this one girl before she realizes that he's in an organization. When he meets up with ML to explain that the youngest ran off with their money, ML sews through the BS and before leaving him tells him "You don't know what he likes or when his birthday is... and by the way he likes eel". So then "brother" starts freaking out and is planning with this other horse bestman gang guy to kidnap uke. The horse beastman agrees, if only he could keep uke as his toy until he's tired of him and the brother does not care so he agrees.
Both of them find out that the uke is staying with the male lead because the "brother" explained what he looks like in his animal form. (Small white dog that horse guys saw ML carrying around)
I'm reading the novel and the "brother" of the uke is a POS