I have a headcanon that Lacus has actually heard about Yuu. But not directly from Mika. He just heard bits from Ferid’s conversation’s w/ Mika but usually he’s described as “angel” or “princess” and then thought to himself “oh Yuu-chan’s probably Mika’s childhood sweetheart how cute who knew Mika had a girlfriend” and at chapter 11 he finally saw Yuu in the flesh and he’s all