Leila (レイラ Reira) was a headstrong and stubborn young woman who spent much of her youth with her childhood friends Ladd Russo and Who, despite the fact that Ladd was from a mafia family and her rich Senator father was trying to stamp out the mafia.
Acting as the leader of their little gang, she and Ladd regularly beat up those who dared to pick on Who; first, Leila would go up and slap the offender, Ladd following with a brick in his hands and brutally assaulting the other person. She would then kick Ladd and say "we're done here!" before he could beat the bully senseless. According to Who, she was the only girl who could just tell Ladd what to do like that.
Eventually, she and Ladd mistook their friendship for love and decided to elope together, even as Ladd became more and more unhinged in front of her eyes. They ran off to elope, but Leila died partway through the process.
How Leila died is unknown, but after hearing Ladd talk about it, Who is certain that she did not die by Ladd's hands. If she had, Ladd would have been "even worse than he is now."
Who thinks that Ladd doesn't like talking about Leila and that he won't bring her up voluntarily if no-one asks.