She's not his mother. Riley knows this, even though it's hard to feel it, sometimes. She takes care of them, growing up, making them stronger. Dr. Walsh is a brilliant psychologist and biologist and who knows what else- she's a polymath, a genius, and inventor. She could revolutionize the world, if only it wasn't so hard to get anything done with maleficaria roaming the earth, killing any children. They snuff out potential, she had said. That's how she settles on her job: finding promising magical kids whose parents won't be able to prepare them for the Scholomance (independents have such low rates of survival, she reminds him), and taking them in as her own. But she's not a mother- just a scientist.
Riley is one of five boys. She trains them all with the hopes of getting them into the Scholomance- but at least, with the hopes of them surviving past 18. It's with the first one, Forrest, that she realizes the major problem- when you use potions to make a growing wizard strong, that doesn't do anything about their personality. Forrest is cocky, reckless, and aggressive; he gets himself in trouble he can't get out of, and he dies alone. Riley thinks that seems obvious, but it's probably hindsight bias; Dr. Walsh is smarter than he could ever be, and she didn't predict it. The second one, Graham, is careful but still has the instincts to act quickly, but alchemy just isn't his strength; he dies when he can't improve enough on the potions she made for him. Dr. Walsh is always perfecting the formula. It's not enough to save Graham.
With the third, Liz, Dr. Walsh trains her in everything that her talents will allow and her situation needs- languages, creative writing and alchemy. Liz is the first of them to make it into the Scholomance, but even though she makes it to senior year, that's the last they hear of her. Maybe she's alive; Riley likes to think she's living a quiet life, using just enough magic to get by. Maybe she's dead, one of the many that fall in the graduation hall. Maybe she didn't make it to graduation- it's not like they would know. Or maybe it's need-to-know; Dr. Walsh doesn't tell Riley everything.
He's got a younger brother, left, Adam. Dr. Walsh is going to send him in next year, right after him. Riley's job: work on improving the potions on the inside, while Dr. Walsh and Adam work on that outside. When Adam comes in with their updates, they can compare and let the cream of the crop rise to the top. Riley wants his formula to win, of course, but it's not about that. It's about him making sure that the both of them make it to graduation. On the day of, Riley has everything he'll need: one of each potion to start, some reagents, a mana crystal, and a kiss on the forehead from Dr. Walsh. And then he's gone.