Hi, I need some advice new to this and dming in general so sorry if I am posting this in the wrong place, so I am running a high school mean girls' campaign (Not the moive also we are a bunch of girls leave me alone lol) that got transported to a fantasy world after they died in a series of events caused by one of their boyfriends Tlyer, (the npc). So we play our first sessions it goes amazing and I ask what they want to see they say they want to see more of Tyler or as they like to call him F686cka$$ Tyler. Soo the issue is I have no idea how to include him. Any advice? 

Somehow, you're going to have to kill tyler so he ends up in the same world as the players.
maybe he got caught by police and got the death penalty
maybe he was actually doing the bidding of some fiend in the real world, and then the fiend brought him here to finish the job.
maybe he was actually doing the bidding of some fiend in the real world, and the field double-crossed him when it was time to collect. 
maybe he's a sparkly lich and was turning the kids into copies of his phylactery to make it harder to kill him, something went wrong, the lich'es soul is now spread out across all the girls and he has to get his sould back.

I mean, it's totally up to you how you eventually explain it.
But Tyler needs to be in the same world as the players to be interesting to them.

And then he's goign to need something in his backstory that has him continue his pursuit to kill the players.
maybe give him some complicated thing he has to do or set up before he tries to kill the players in game.
so the players aren't constantly running from Tyler. 
But he shows up as a recurring boss.

Kinda depends on how long you plan on running the campaign.

if its short, just have Tyler constantly come after them.
if its a long campaign, probably need to make Tyler scarce and have him attack every once in a while, to allow the players to deal with other stuff.

I love this thought I should have made it clear in post that he killed the party by mistake by jumping into a roof off the pool and the chain reaction killed my two players one of them choking on a cherry and the other cushioning his fall though him being evil is great idea I am gonna use that tysm 

I would not make him evil.  From what you have said here, he appears to be a bit of a buffoon that your players have gravitated to and expressed an interest in being more involved in the game.  That makes for a great bumbling character that causes problems every now and then, constantly has to be rescued, and is generally just an entertaining bit of comic relief.  Once he crosses the line into evil, you are sort of telling players that he is no longer the "haha, what a moron, he's fun" kind of character, but "oh, now he is something we have to deal with."  That does not mean there are not ways to do that properly - but this seems like a bit of a more lighthearted silly campaign based on the limited data available here, so you should lean into that with the character they have already decided is fun. 

Yeah I think I might do that because I played him to be really dumb tysm for the advice 

The fantasy place you place the characters can have doppelgangers of a lot of the same people from where the original PCs came from.

The character could have shown up because their doppelgangers disappeared (need to back track a little with artistic license).

That can give you an instant credibility of bringing that NPC as well as as any others e.g. parents/school friends/co workers/etc.  This gives you the chance to imitate any one you group life.  A doppelganger does not need to be an exact copy and can be more extreme (good for artistic license) to allow all of you to have fun with a know NPC.

A good heel-face turn is always fun. He starts out a doofus/jerk, kills tye party by accident, and maybe shows up and screws up the players plans accidentally once in a while at first. But maybe he disappear for a couple levels and when he shows up again he's competent and maybe found a way back to the real world, and came to let the playere know.

So Real World Tyler did stuff that got the party trapped in a fantasy world after real world dying.

I'd say just keep letting the party bump into people who know "Tyler".  Start small, with like maybe someone from town also meeting an untimely end due to Tyler's shenanigans.  Then later someone who's been "trapped" for more than a decade who blames "Tyler".  And all they keep finding out that "Tyler" has been "accidently" killing people for centuries and this entire pocket dimension is more or less his creation.

Slow and steady burn from bafoon to BBEG.

I love this!

been brought up before, but i am going to place a twist on it. 

Make a lot of people trapped here because of Tyler, make him sound like a villian, then when the real villian shows up, he has a piece of Tyler's soul, and has been making him a Buffoon with lethal consequences for years to bring people to this world for some reason. 
Make it so the players can get that part of Tyler's soul, then you have a Ghost NPC who they can talk to, but can't make do anything, so they don't start worrying he will be a DMPC.

You can even have some weird lore/magibable reason like "The hole that is in his spirit sucks in the souls of those he thinks about or die near him." or like "His family is full of necromantic potential, but he is unaware of his gifts, so he has no way to stop me when i force him to use them for me in your world."

Just some ideas, go ahead and play with them.