Friskriel isn't an official character in 'Undertale'—the name seems like a fan-made mashup of Frisk and Gabriel (possibly referencing the angel Gabriel?). Frisk, of course, is the game's silent protagonist whose choices shape the story. The 'riel' part might come from fans blending names for AU (alternate universe) content, which is super common in the 'Undertale' fandom. There are tons of AUs like 'Underswap' or 'Storyshift' where characters get reimagined, so Friskriel could be someone's OC or a variant from one of those.
I love how creative the fandom gets with these spins! If you stumbled across this name in fanart or a fic, it’s probably from a niche AU. The cool thing about 'Undertale' is how it inspires endless reinterpretations. Even if Friskriel isn’t canon, the idea shows how much depth fans find in the original characters.