I have both Themis Lyra and Anne Moony to offer to the cause!

Themis was an orphan herself and would have a serious soft spot in her heart for this kid. I can offer her as a teacher would would take an interest or could figure out how hard of a time he's having in class.

Anne would feel an affinity as she's one of the only other Slytherins that doesn't come from money. She would look out for Leo, in her own way. She could frustratingly attempt to tutor him, or wouldn't have issue slipping him some of the money she was going to use in Hogsmeade.