Just got the headcanon of dudley being super afraid of his parents, so when Harry is attempting Hogwarts he runs away from the Dursley becouse he thinks they would treat him as bad as Harry, now that he is gone.
So when Harry comes home for holidays and Dudley isn't there he first doesnt really care, but after three das he gets suspicius and asks the Dursleys what happend but they just tell him to go away.
So that night He sneeks out and calls all of Dudleys friends but noone can tell him where he is. Super worried Harry runs threw the night trying to find him and eventually does.
Dudley is super happy to see him and when they get home dudley hugs him and says "dont leave again, you know how they are" and then goes to his room.
The Dursleys find out and cage Harry in but not dudley since He is their prince.
So when Ron, Fred and Goerge come to the windows to rescue him he sais that he cant leave dudley here and Ron asks why since He only always has been rude to Harry but Harry Just simply sais "you have no idear how they can be"
So he runs and gets Dudley who is half asleep and Has no idear what's ging on but imedeatly follows Harry as he sais that he is going to run away forever.
Harry and Dudley sit in the back of the car together with Ron. Dudley still being sleepy leans on to Harry to sleep and neither Ron ore fred & goerge can believe it but Harry just tries to stay calm.
At the burrow Dudley is wield awake and super exited to be in a flying car with wizards.
Once their in the burrow with Molly she is super sweet to Harry but just gives dudley a harsh look becouse she heard what that boy did to Harry all his life.
Dudley is still super tierd so Ron lets him sleep in his bed after a long discussion with harry.
At the table Harry asks Molly if Dudley could stay and sais "He might be a brag, but he is easy to hadle. Im Sure he can help out" so Molly sais yes and dudley stays at the burrow.
He goes to Plattform 9 3/4 and Diagon alley with all the Weaslys and just falls in love with everything magic can Provide.
Over the years He and molly grow to be super tight and Arthur sonetimes takes him to work and Shows him everything while asking dudley questions about muggle.
Over the years he grows to be verry caring and loyal towards the Weaslys. He doesnt eat that mich and gets down a fey pounds while getting some muscels from the work he does, even through Molly tells him not to since they could just use magic, but everytime she sais that Dudley Just response with "the muggle way might be hard but sonetimes it is much more effective, and where would we get to if we would use magic fore everything" wich Molly just loughs off.
At Bills Wedding he helps molly to plan and decorate everything. He dances with her, Ginny, Fleur and Also Hermione wich is the time he tells her she was waaaaay to smart to be friends with harry.
When the burrow burns down Dudley makes sure everyone is outside and tries to safe mollys knittig stuff.
When the war comes he wants to help so he becomes a muggle spy and even learns some of the muggle triks to fight off wizards, wich he then trains with Arthur and Molly.
He doesnt find out about Harrys death after the battle and is super mad at everyone for not being told emediatly, wich molly just gives a sideglance and sais "you would have gone nuts kid". He was also last to find out about Fred and Percys betrail. At the funeral he goes to him to really mess him up but finds this broken boy who can't cry even tho he really wants to. So since he basically been raised by Molly He gives him a bonecrushing hug an tells him everything would be just fine and "I understand how you feel but you are good they know how sad you are. They See your pain and hear you suffer. They will forgive you, there family after all" wich he sais while he glances ofer at Harry being just super thankfull for not leaving him at that horrible place he used to call home.
A fey das after the war Dudley asks Harry if he could take his Name so Potter. To Harrys questions why He would want to do that he awnsered "I have the Name of a memory one that I wich to forgett." he then gave him a bonecrushing hug much like Mollys and said "my brother nearly died. I dont want to be seperated from you ever again."
He marrys a muggle and raises three Kids two of whom are wizard they call them Daisy, Arthur Ray and Precy Fleur only Arthur turns out to be a muggle. For both namens Arthur as well as Fleur he asked the Weaslys fore aprovel. Fore Precy who was originally named after Percy he only asked him.