You replace one of the people that already existed on Earth Bet. This includes any of the Original Companions, provided you pick the option with the appropriate strength level, even if they may not appear in the story normally. You keep their power if they are already triggered, in addition to any powers you choose here. You shall have all their memories and skills, you'll be able to act just like they did. You can decide how far you merge, if it's just your mind in their body with their memories and skills, or if your personalities merge completely. If you want you can decide how this works if you want to figure out a way to make this not stealing someone's body, mind, and soul from them. If you Self-Insert into someone who has yet to Trigger, if you Trigger or Second-Trigger you will gain their powers.