Introduced by series producer Matthew Robinson, the character Dan was an alpha male hardman who originally came into Albert Square as the new boyfriend of local resident Carol Jackson (Lindsey Coulson).

Their relationship escalated when the two got engaged and she became pregnant with his unborn child, but it soon transpired that Dan had a previous fling with Carol's eldest daughter Bianca (Patsy Palmer) and the pair ended up having an affair despite the latter's established marriage with her beloved husband Ricky Butcher (Sid Owen).

The events of their competitive rivalry initially started out as a close friendship and business partnership when Dan became co-owner of The Queen Victoria public house after Phil sold his family's share of the pub to him, which in turn led to Dan clashing with both Phil's mother Peggy (Barbara Windsor) and also Ricky's father Frank (Mike Reid) on multiple occasions.

At the same time Dan established a major conflict with Phil's fellow archenemy Steve Owen (Martin Kemp), which had been developing in the case of both Steve and Phil having romantic connections to Dan's former girlfriend Mel Healy (Tamzin Outhwaite) after his brief romance with young neighbour Teresa di Marco (Leila Birch) didn't last long.

It was then Dan's story arc concluded when he kidnapped Mel to successfully blackmail both Steve and Phil into giving him a £200,000 ransom, after which he ultimately left the country.

Dan Sullivan arrives in Walford — a fictionalized borough in East London — on 7 June 1999, appearing as the latest boyfriend of local café manager Carol Jackson (Lindsey Coulson).

They soon move to Walford's local community, Albert Square, where the couple plan to get married after Dan learns that Carol is expecting his child.

Their wedding plans surprise Carol's daughter Bianca (Patsy Palmer), as she and Dan had a fling when she was a teenager on holiday many years before.

It is at this point where Dan has become closely acquainted with Phil Mitchell (Steve McFadden) — the square's local hardman who is also partly related to the Butcher family as he is Ricky's former brother-in-law and Frank's eldest stepson.

Towards the Millennium celebrations, Dan finds Phil spending his Christmas alone at The Queen Victoria public house and comforts him over his wife Kathy Beale (Gillian Taylforth) and their son Ben leaving the county without him months ago.

They soon proceed to form a business partnership when Phil sells his ownership of The Queen Vic to Dan for £5; with Phil having done so to spite his mother Peggy (Barbara Windsor) in retribution for supposedly favoring his brother Grant (Ross Kemp), who recently left the square following a botched criminal job between the brothers, more than him.

However, Dan refuses to relinquish his ownership of the pub back to Peggy and later discovers that Phil is running a car lot scam with both Frank and their colleague Roy Evans (Tony Caunter).

Phil backs down and leaves Dan outside of the hospital, where inside he finds out that Lisa has suffered a miscarriage and that their unborn baby has died.

Their growing relationship is initially contracted when Dan befriends and later dates one of the pub's regular customers, Teresa di Marco (Leila Birch), which causes her overprotective brother Gianni (Marc Bannerman) to become suspicious over his intentions.

Mel subsequently confides this secrecy to Lisa, who ends up telling Phil about Dan tipping off the police about his motor scam.

Moments later, the police arrive thanks to Phil's tip-off and Dan is consequently arrested; he is later charged with attempted murder and will be put on trial for the crime in July 2001.

Before the trial commences, Phil convinces Steve to work together in getting Dan imprisoned — under the guise that the pair, though archenemies, would be well-ridden of their common nemesis and Mel's tormenting ex-boyfriend once and for all.

Phil also summons his lawyer Marcus Christie (Stephen Churchett) to help Jamie prepare in testifying against Dan, while also forcing Ian to help contribute to his testimony as well.

Dan soon kidnaps Mel and holds her captive in an abandoned tower block building, whereupon he alerts Phil and Steve of the situation.

After forcing Dan back into the room where Mel is being held captive, Phil is unable to find her and demands to know where she is kept hidden.

After stating they are now even, Dan gets his final revenge on Phil by gagging him and leaving him handcuffed at the radiator — bidding his archenemy a morbid and taunting farewell.

Though he is not seen again on the square again, Dan leaves flowers on Steve's grave following the latter's death in March 2002 — at the epic climax of his feud with Phil — bearing the message "Gotcha!".

[2] The possible suspects were Lisa Shaw (Lucy Benjamin), Melanie Healy (Tamzin Outhwaite), Steve Owen (Martin Kemp), Mark Fowler (Todd Carty), Ian Beale (Adam Woodyatt) and Dan.