I have a character in my novel, De-Shi, who's the mentor and love interest of the protagonist, Li-Mei (nicknamed Hanging Pup). He listens to her feelings, gives her advice, jokes with her, shares philosophical thoughts with her. Anyway, here's a taste of their dialogue:
De-Shi sighed. "How does your boyfriend stand going on vacation with you?"
"I don't have a boyfriend," I said. "And I don't go on vacations."
"That explains your good temper ..."
"No one forced you to bring me along."
"Did you know that anger is the direct product of fear?" De-Shi said. "Animals developed this response to get rid or stay away from possible treats. Same for humans. Imagine what'd happen if we didn't feel rage toward a killer."
“You okay, Hanging Pup?” De-Shi’s voice came muffled from behind the sheet.
“Yeah, I ... think so,” I answered.
“I know how you feel. It was the same for me the first time.”
I sat up and turned towards him. “Really?”
A short silence. I imagined De-Shi nodding his head in the dark. “You feel as if something has crept inside you. Something dark and murky, like crude oil. It travels gradually, silently, polluting every cell, every nerve, every organ in your body. You can feel it. You can feel it eating you inside. And the worst part is, you’re the only one who’s aware of this. Yes, you can explain it to people, but just skin-deep. They can’t help you. So you have no choice but to fight alone. Fight this darkness alone.”
But some readers say that they felt a "lack of connection" with De-Shi (which also affected their interest between Li-Mei's and De-Shi's romance).
Here I quote one of the readers (he just read Chapter 10):
I don't know anything about him other than he started this club. I assume since he was in that class where Li-Mei asked her question he is a veterinarian science major or something like that. But regardless, I never felt the chemistry between them.
They have a similar interest, but it seems when De-Shi is talking, it's more like reading from an encyclopedia then a person sharing something. We need to flush out his personality. Why does he know what he knows? What is his major?...
Have him tell a story that will endear him to the reader...
This is something that I did intentionally. I wanted De-Shi to be funny, scientific, philosophical. And make him talk about his past only in the last chapters. (Guess I wanted to do that to make him a bit mysterious.)
So my question is, how can I make this character who doesn't speak about himself relatable?
EDIT (based on ShastriH's answer):
The other characters do talk about De-Shi:
"Don't worry, kiddo. I'm a tomb." She patted my arm. "I suggest you take the initiative, though. The guy might be an animal expert, but when it comes to human courtship, he's totally clueless."
This doesn't happen too much, though.