Chevalier was adopted by Éclair, as she remembered in Episode 10. The same memory can be seen from Chevalier's angle in a later episode. As much as we know and what the two of them talked about, they were happy as a family.
However, as the government realised that Chevalier was actually a Nouvlesse (Pure blooded human), they took him away from Éclair. She said that Chevalier will eventually forget about her, as Chevalier was only about 5 or 6 when the government's agents took him away. However, Chevalier promised to remember her no matter what.
Chevalier became the head of GOTT in order to protect Éclair no matter what happens. Somewhere between him becoming the GOTT's head, he was allowed to access Éclair's files, and that's where he found out about Éclair's ES membership. However, when he finally got to see Éclair again, it was past when Éclair deleted her own memories; including the one the two of them shared.