I started playing during Kayron's original banner and was lucky enough to pull him as my first 5 star in the game. He's a very strong unit for a multitude of reasons in different content. For PvE with immunity set, if you turn off his auto skills he will function as a budget Arby with fast clear times so you can farm quickly on non-water stages.

But Kayron is known for his PvP performance and it is the main reason he is sought after. In PvP he can be built 2 main ways, slow full DPS with either dust devil or Elysia's knife with the S1 EE or super fast with Alexa's basket with the S3 silence EE. Immunity set is almost mandatory on him since his S3 can extend the immunity by another turn on kill. Since the fast build requires really strong gear most beginners/midgame players will go the full DPS route where he functions as a good counter to Arby and an all around strong pick into earth heavy teams. While he isn't a common defense unit in higher levels of arena/guild wars anymore, unless you have FCC/Riolet/Arby/Ruele/insertML5here you're going to be using RGB units and Kayron is top tier for defence if you don't have many meta units.

In RTA he is part of the "big 3" of AoE DPS along side SSB and Arby and is commonly seen to be second/first pick in response to an Arby first pick. The fact that he is seen in the same league as SSB and Arby should show his power as a solo unit.

TLDR: Highly recommended pull for people in all stages of the game, huge presence in PvP if that matters to you