Selora: Her best friend is Viago. Although best and friend are both pretty big stretches in terms of how they actually treat each other. There's a lot more balance between them after Veilguard, but it does not change that they're both pretty horrible at friendship. They do, however, know each other better than anyone else, as much as that can be true for either of them, just by virtue of having been close for as long as they were. Lucanis is a close second, and part of why she struggles so hard with the fear that she will lose him, but their relationship is complicated by also being romantic. Next to either of them is Neve and Davrin in terms of who she got close to during the course of Veilguard and who she remains close with after.
Leithe: Her best friend has only ever been Davrin and that's really only true because he was persistent about being her friend at all. She did have other friends as a warden but most of them were because of Davrin, anyway. She did get pretty close to Taash while living at the Lighthouse, but all of the team are on the same level of being people she cares about. While she's in the Fade post-canon, she does often dream of their time in the Lighthouse like they're still there.
Selora: Her first and only relationship before Lucanis was with a barmaid in Salle named Letta who was a few years older than her. They used to work together to cheat gamblers in the tavern (some of you might recognize her from Opportunity Makes the Thief) but they weren't all that serious. Rather, young Sel was not all that serious and avoided most relationships that defined themselves as such. She's still good friends with Letta, though. Letta owns her own tavern now and Sel stops in to hang out with her whenever she's back in Salle :)