heir to kurain, queen of food
Maya Fey is the person that matters most to Mia, and though it's not quite the same for Diego, he still does a lot for her, up to and including murder. In turn, Maya cares deeply for her sister and eventually understands why Diego acted as he did.
Mia
To Mia, Maya is the one thing she must protect more than anything else in the world. It's for Maya's sake that she leaves Kurain Village, not wanting their relationship to end up like her mother's and aunt's did in the battle for the title of Master. By leaving, Mia saves her sister from that pain and can foster a strong relationship with her. What she can also do is search for the man who caused her mother to go into hiding, Redd White. In doing that, she protects Maya from the eventual discreditability she will face when she takes on the mantle of the master of Kurain.
In turn, Maya believes in and looks up to her big sister. Mia is who Maya strives to be like — not so much that she wants to become a lawyer, but to be strong like Mia. It's a hard task for someone as happy-go-lucky as Maya, but the trials and tribulations she faces make her grow up. What Maya doesn't realize initially is that her role is the same from Pearl's perspective. Eventually, though, she learns that she has to show a strong face even when she's broken up inside just like Mia did for her.
Diego
Much like the rest of the cast, Maya is rather confused and intimidated by the mysterious Godot. It's in the last case, after Dahlia leaves her body, that Maya understands who Godot is and where he's coming from, and her reaction is that she needs to protect him after all he's done for her. Phoenix gets the truth out of her, but at the cost of what that truth reveals: that Godot killed Maya's mother to protect her. That much, Maya understands, and doesn't blame him for it; rather, she regrets that she couldn't protect him.
But did Diego even protect Maya for her sake? Not really. Diego himself says that he had only two reasons to live when he woke up and realized Mia was gone: to get revenge on Phoenix for letting Mia die and to protect Maya the way he hadn't protected Mia. But as he himself realizes, he didn't really do it for her, he did it for himself.
Godot: I suppose... I wasn't really interested in saving you at all.
Maya: Huh...!?
Godot: I think I was just trying to salvage what's left of my own broken soul... I was trying to make up for the fact that I couldn't save Mia. Nothing more.
If Diego had really wanted to protect Maya, the person he should have gone to was Phoenix. However, his pride made that impossible, and ultimately resulted in tragedy. Maya probably finds the loss of her mother easier to deal with because she hardly knew her mother, since she'd been missing for so long and didn't even try to talk to Maya directly. As such, it's more like a stranger's death, and so her forgiving Diego isn't too much of a leap. Had it been someone closer to her, she probably wouldn't have taken it so well, all things considered.