You're the first to ask and he's not part of Underwood canon, so this answer is gonna be all the publicly available information about him so far!

Lars, who later chooses the name Anthony when he realizes he's trans, is the youngest of the three Strathmore children - younger than Ridley by ten years. Sexually abused by his father Wendell from an early age, most of Anthony's childhood is a blur to him. He remembers very little about Ridley, especially since Ridley left home when Anthony was only five. He idealizes Ridley not only as a concept of masculinity who isn't his dad, but as the one who managed to escape the incredibly bleak environment that is life with Wendell.

By the time he's old enough to understand what's happening to him, Anthony has already resigned himself to essentially filling the role of Wendell's wife - keeping him happy, sexually satisfied, emotionally regulated, and generally looking after all the man's needs. He loves his big sister Abigail, who works very hard to get an education so she can provide for herself and Anthony without needing Wendell, but he doesn't really believe in that future. He can't imagine a life outside of his father's abuse, and loves Wendell too much to want to abandon him.

However, one day when Anthony is fifteen he wakes up and finds that Wendell died next to him in the middle of the night. Suddenly Abigail inherits everything, including the status of being Anthony's legal guardian, and she reaches out to Ridley to invite him to Wendell's wake so the siblings can reunite. Anthony, while still deeply grief-stricken, is thrilled to see Ridley, and they quickly bond.

Unfortunately Ridley's been deeply scarred during his time away. You already know about that, I think - the grooming and abuse from Alec, the incident with Isabelle, everything that Grayson did to him during Ridley's time as his slave. He doesn't really know how to express love and be close to someone without an element of sex. Anthony, too, has already been groomed by Wendell into thinking that part of love and looking after a man that you look up to is sex. You see where this is going.

Ridley eventually realizes that what's going on between them is wrong to say the least, and has a crisis of conscience over it. He sort of "breaks up" with Anthony, apologizing to him and putting some distance between them for a while. Anthony is deeply hurt at first, but over years and with some pretty intensive therapy he comes to understand that Ridley made the right decision and that they never should have had a sexual relationship in the first place. At the same time, he can't bring himself to resent Ridley, and they eventually grow close again when they're both adults and try their best to have a normal brotherly relationship.