Origins/Training saga summary
You fall into the underground 2 months before gaster fell into the core and 6 months before chara. You were dying and to make matters worse your soul was cracked. The dreemurr family is off doing important business so Asriel won't be there to save you. Luckily you were able to get up. You knew, when you got up, that this was the undertale universe or seems to look like it. So with the last of your energy, you teleport into the throne room then pass out. The Dreemurrs and Gaster help you but you are dying. So, to save you, Gaster gives you part of his DNA making you his son and part monster.
Since have You have a monster soul your Determinate is weakened but Magic is stronger. However, there are 2 ways of getting it stronger. 1, injections of Determination, and 2 Gain LOVE. You didn't need Determination at the time so didn't do either. When Gaster fell into the CORE, and only you and Sans remember him, You spent less time with your bros and time training for flowey and eventually frisk.
You found out that you can use the powers of other soul traits. You found out that you can use Hate. You got efficient with every soul trait except for 2 Hate and Overwrite. You're as strong undyne the undying not serious. You stopped train because it was 2 months before chara comes and build better relationships with everyone. You and Pap are pretty close. Your Relationship with sans isn't bad but it could be better. You became good friends with everyone except jerry.
Life was Good But it was about to change for the better or worse.
Chara saga
Everything when Per Canon to undertale Except you and chara Became friends. Also, What do You guys want about Chara's Gender? The next Chapter Will Be the Flowey saga.