Posted in rules and mechanics forum as well because I'm unsure which forum this was best suited to.

Hey guys :)

First off, I'm fairly new to DnD (1-2 campaigns, a few months at most) and I am brewing a new character and while my party and I love the concept, we're worried that simply allowing it would result in it being hellishly OP.  So I thought I'd come here and see what you guys thought - I'm looking for suggestions for how to make this balanced or what drawbacks I could use to have this be fair strictly from a mechanics perspective, I'd rather not rely on role-playing the character in a specific way as a solution because it can be difficult to judge.  I also don't currently have a backstory or race chosen for this character and am learning towards him being a sorcerer with warlock as a secondary option.

Backstory:

As a young child the character was often fast and moved quickly often being curious about everything and running to investigate without hesitation.  This all turned south when during an otherwise harmless investigation of random shiny object he was attacked by a magical monster and sustained heavy, unrecoverable damage to his legs making him functionally immobile.  Because the child was young he still wanted to explore but obviously couldn't.  That was until one day, when his 3-year old sanity was at its wits end, he moved without moving... simply slipping from his current position to where he wanted to go, despite his legs still being unable to walk.  From that day forward he learnt how to use this odd power to move himself around, reducing the time between blinks as he grew older.  In the current day, he can be found sitting on a plain wooden stool which and wears midnight black trousers with a magenta blazer and mild orange shirt underneath.  Not many see him for more than a few seconds and as a result, he exists more as a myth than man.

TL:DR: the character had his legs messed up as a child and learned to channel magic to teleport short distances at will to functionally replace his ability to walk.

The core of the rules stuff that I've identified so far:

Obviously, because of his condition, his speed would either be 0ft or 5ft (because I can imagine he'd be able to shuffle his chair although how fast would be up for debate) and the teleport would function similar to misty step wherein you could only blink up to 20-25 feet and only to a location that he could see (so no teleporting through walls) except it's instantaneous and no cloud of myst.  Additionally, the act of blinking costs him the same amount of energy as it would for him to walk there so he can't just blink endlessly or too quickly or he'd kill himself/pass out from starvation/exhaustion and it makes crossing something like a ravine difficult.  To top it off the way he moves within combat poses a problem because in his current state he cannot be the victim of opportunity attacks but it would also be assumed that due to him always being seated, he'd also not be able to make opportunity attacks should someone decide to run away.  This does also mean that if he were to wield a dagger, almost all of his melee attacks could be done whilst flanking from behind which could pose a DPS problem if he were to choose rogue once we get a high enough level to multi class properly.  There was also mention of him always being in quarter-cover whilst sitting but I argued that because he's always seated, he is standing as tall as he can and should be treated more as a small character than a medium one because of it.

Potential solutions:

 - set a skill (dex) to be capped at 9

 - have his default proficiency bonus to be 0

 - have him take double damage from flanking attacks

 - have his hit dice be 1d4 to compensate for the fact that he's hard to pin down in combat

 - have a natural -3 to initiative 

 - Make him unable to use most weapons, in general (except for small weapons like daggers, etc)

 - AC would be 10 or 11 because wearing armor whilst sitting would get very uncomfortable after a while

I suspect that the actual fair solution will need to be a mix of some of these elements to make sure that the instantaneous movement isn't a problem for gameplay.

Any and all ideas welcome, here's hoping we come up with a solution! :D  Also: if this post needs to be moved for whatever reason, lmk