So my party came across the DoMT and my 6th level half-elf fighter drew the Knight card, a level 4 fighter is now loyal to him until either dies. However, the next card my character drew was the Dunjun card so I disappeared into an extradimensional sphere leaving everything I was wearing behind.

My plan was to play this fighter whose backstory is literally "Popped into existence to serve my master who has disappeared before my very eyes only seconds after meeting him. I must find him and rescue him at all costs." However, I have no idea how to turn that into a playable character.

Any help would be greatly appreciated.

EDIT: I realise It's called the Deck of Many THINGS, not cards. Don't come for me.

Played with fire and got burned.

First decide the fighter's subclass. Then if you still have the deck, toss it (it is not worth it).

I suggest playing the character with no personality except to serve a (now missing) master, and have them develope a personality over time.

"You appear in a strange location, you see are dressed in plate armor, have a shield and longsword in you hands, and you see before you a half-elven knight holding a strange deck of cards. You feel a strong connection to the knight, as if he is your master, and you must follow all his commands. He draws another card, which has a dark staircase leading up to a caged door. Dark energy wraps around the elven figure, and you see as they are ripped out of reality, disappearing to an unknown location, or just being completely erased from existence as the magical card disappears into thin air. You look around, see a party of adventurers, covered in slashes and bruises. You ask them what happened, and they explain to you that the half-elf knight was a member of their party, but was taken to a different plane of existence, and they have no way of freeing him."

Your character obviously is confused, you didn't exist before, but you suddenly do, and your master who created you is now out of reach, you long to be with him and do as he asks, but he can't be freed, unless you gain a wish spell. You know the power of the deck of cards that lays on the ground in front of you, you know it may have the power to free your master. Your character would likely be obsessed with this deck, would want to take it and draw as many cards as you need until you get a wish spell to free your master, and if you draw a card that would hurt you, why would you care, you have no reason to believe that you have a soul and would only suffer for a small while until you died and had your essence return to the deck of cards. 

Your character would want to free your master, you would likely be adamant about using the deck of many things to free him, but if you couldn't do that, you would look for a different way to free him. You would know that the party members are likely going to have the same goals that you have, and your best option to free your master would be to stick with the party members, because even if they don't let you draw from the deck of many things, you might be able to find a different magic item to free your previous master. If you get to higher levels without the ability to free your previous character, your new fighter might accept your previous character as a god or otherworldly powerful force. This would likely come about because of this:

1. You were created by your previous character, which means that you would likely see them as an infinitely powerful force that can create life. Even if your party members told you that they were just mortal like you are, you likely wouldn't believe them because you yourself aren't wholly mortal (you were created completely by magic), you have undoubtedly heard tales of your previous master's accomplishments, the lives they saved, the monsters they had slain, and anything else special that they did. 
2. You also would have nothing else to attach yourself to, you wouldn't understand normal gods, so you might attach yourself to your previous character. (maybe multiclass into a cleric, paladin, or warlock that believes they're pulling their magical powers from your previous character, when it is likely a different force, maybe a real god that you are misunderstanding as a banished fighter.)
3. You feel an unbreakable connection to an otherworldly being that created you, and you must serve them, but they can't communicate with you directly. Doesn't this perfectly explain the basis for almost every religion on the planet? Try using a sending spell to contact them, that might work.

Your character would have a personality unlike any other in D&D history, congratulations, you have the ability to be a completely unique character. These are just a few character ideas, if you don't like them, that's fine too.

Hey, I know it's not likely for a reply, but any update on this? I would love to hear how it turned out.