This is super overdue since he’s technically a main character but here’s some bits and pieces!

• Valerian’s parents are two of the gijinka/turned-human Legendaries – specifically Darkrai (Yuri) and Cresselia (Theia).
• In their lives before being deified, Theia was a noblewoman in roughly 18th century poké!Russia and Yuri was a thief who she fell in love with, despite their different lives. After turning back to their human forms in modern Sinnoh, one of the first things they do is find each other and reunite, wanting to make their relationship official after not fully getting to before. Theia becomes pregnant pretty quickly.
• Either right after Valerian is born, or shortly before, Yuri is captured by Team Oracle, who are targeting former Legendaries. After losing her husband/partner, Theia decides to go into hiding to protect herself and their child.
• Valerian is raised in some cabin/house in the woods in a remote part of Sinnoh, with the only adult around being his 18th century noblewoman mother. As a result he ends up a bit… different. He’s very old fashioned in his mannerisms and is pretty sheltered. 
• He’s not completely sheltered though since his little family had a connection to the outside world through Tobias, who had been friends with Darkrai and continued to be friends with Yuri after the Legendary reverted forms. He would often visit Theia and Valerian’s house and bring supplies, toys and games for Val, news for Theia, etc. He even brought Raphael to visit a few times, so he’s maybe the closest thing Val has to a younger sibling, as he didn’t get to interact with other children much apart from the times he’d go with his mother to the nearest (very small) town to buy groceries and the like.
• Once he’s older (around 14 or 15, so a bit older than the norm for newbie trainers), Val sets out on a journey despite his mother’s protests, wanting to find out what happened to his father rather than just live in hiding. As a result, he’s very active in investigating Team Oracle. At the same time, he also decides to challenge the Sinnoh League since it was something he always dreamed of doing from his childhood. His starter is a Starly he befriended that had been living in the woods near his home.
• At some point, while raiding a Team Oracle base, Val finds an injured Type: Null who he rescues. Most of the information and data regarding the chimera had been taken with the fleeing researchers or destroyed, so Val had no idea what the pokémon even was. Nonetheless he nicknames the Type: Null Berkut (golden eagle) and does his best to take care of them. Berkut eventually evolves into a Silvally under Val’s care and the two are very close, though Val still has no idea what kind of pokémon they are until he meets Gladion in Alola. Finally having answers is a breakthrough for both of them (and Berkut is happy to meet another of their kind!).
• He’s Emery’s love interest! Though due to him being kind of dense, nothing really happens until close to the climax. She calls him Valera as a nickname which he finds adorable but is too embarrassed to admit it.