Born in Ireland, 1727, as Liam, the elder son of a middle-class relatively well established family. In his human days, Liam is mostly a drunken layabout, spending his time drinking and whoring, a terrible disappointment to his parents (in his father's words). No decent side of him was ever shown, but we know it existed, if to judge by the way he treated his little sister Kathy.
In the age of 26, 1753, in an alley behind a tavern in Galway, Liam meets Darla, a 146 years old vampire and that night his life changes forever. She offers to show him a world he's never seen. He accepts. That night, he dies, and the demon later known as Angelus is born.
Angelus' firs kill occurs as soon as he rises, and the victim is a graveyard keeper (I think?). Later on, wreaking havoc throughout his home village, he also makes a quick stop at home to murder his entire family. He manages to get into the house in the first place because his sister lets him in thinking he's returned to her - an Angel. In fact, Kathy names him, he later embraces the name Angel in Latin - Angelus, and is presented this way before the Master in London, 7 years later.
After disrespecting and mocking the Master of Aurelius and refusing to serve under his command, Angelus leaves with Darla and they start out on their own. Very quickly, Angelus gets the nickname "The Scourge of Europe", and said nickname is rightfully earned. He and his Sire murder their way throughout Europe for 150 years, leaving their bloody mark in every possible place and very rarely someone to tell the tale. And still, they live in style. One of the things that stood out in Angelus unlike in most vampires was his style. He even *kills* in style ("A good kill, a real kill, takes pure artistry. Without that, we're just animals" - Angelus, "Fool for Love"). But…one time, he kills the wrong girl. 1898, Romania - Darla brings her to him as a present - a Gypsy girl. Angelus raps, tortures her and kills her, which according to Darla later on, doesn't take longer than an hour. But it isn't the end of it. The girl was the beloved daughter of her clan, and they punish him - they give him back his human soul.
Ever since, he is doomed to suffer for all eternity, to remember every single crime, every single murder he'd committed in 150 years, remember all his victims - each and every one. He tries to return to Darla at first, but she wouldn't have him, she refuses to have anything to do with him while he had a soul. He has no choice but to close that chapter and leave the life with his Sire behind him.
Angelus immigrates to America in the beginning of the 20th century and changes his name to Angel. There, he separates himself from all vampire population and never hunts again. He lives in privacy, shunning humans and vampires as one. What we know for a fact from that period of time is that around the 1950's he resides in the Hyperion Hotel in Los Angeles, California for some time, and that is when he first, after years of solitude, dares to make a connection with humans, to help them. But they betray him. Said betrayal cuts deeper than imaginable and eventually leads him from a relatively decent living to the dirty back streets of Manhattan, New York, when he is living as a homeless man, feeding on rats, that's when he can catch any, and barely standing on his feet.
In 1996 a good demon that works for The Powers that Be names Whistler finds Angel and picks him up from the streets. He has a proposition for him, he offers him to become someone - by that, taking on himself the role of the protector of this generation's vampire Slayer, Buffy Summers. Angel goes to LA, following whistler's advice, and watches Buffy being called by her first Watcher, Merric. He instantly falls in love with her and accepts Whistler's offer.
For the next almost four years, he enacts as Buffy's protector and lover. After skulking in her shadows for a year, the Slayer moves to Sunnydale, and he follows her and they meet for the first time. She falls in love with him - after 244 years, Angel finally finds his soulmate. But the best things in life never last. On Buffy's 17th birthday, they make love for the first time and the Gypsy curse, which apparently has a 'true happiness' clause, breaks, he loses his soul and reverts back to Angelus.
For months, he tortures Buffy mentally rather than physically and murders people close to her, until they are forced to meet in a final battle - it's either he, or the world. After months of not being able to kill the demon wearing her lover's face, Buffy must, only that she now has to kill *Angel*, not Angelus - his soul has been restored again by Willow. Having no other choice, she sends him to Hell (his blood only can close the vortex Angelus' blood opened.
After months in earth time and centuries of torture in Hell time, Angel returns to earth, to Sunnydale, and with Buff's help slowly gets back to his old self. However, he understands they can't be together, no matter how much they love each other, and in 1999, he leaves his love behind, moving to LA.
In Los Angeles, he teams up with Doyle (a half-demon who receives messages from the Powers) and Cordelia and sets up Angel Investigations, an investigation agency that specializes in 'special' cases (aka, demons, monsters, vampires…list is long, everything normal PI's don't do). Apparently, Angel is a Warrior or the Powers that be and has a large role to play in the world and in the final battle - The End of Days. The way Wesley (who joins AI a little while after Doyle's self-sacrificial death) translated the scrolls, the word 'Shanshu' regarding Angel means life - Angel is prophesized to become human, it's his redemption, which he will earn after his destiny as a Warrior will be fulfilled, after The End of Days.
In 2000, AI move their business to the Hyperion hotel, which has been standing abandoned for over 20 years because of a paranoia demon. They kill the demon and stay in the hotel, where Angel also lives, after his former apartment (and their former office) was blown up.
That year, Darla returns to his life (after he'd staked her for Buffy back in 1997). She is resurrected by Wolfram&Hart as human, to drive Angel to the point of insanity and from there to the dark side, to bring him closer to Wolfram&Hart. The plan fails, even though t works well enough to practically drive Angel crazy, make him renounce his mission, stray from his path towards redemption and fire his friends. The Darla episode wraps itself up with Angel giving in to darkness and despair and sleeping with his Sire, risking the thing that's most precious to him and not caring - his soul.
But he doesn't lose his soul, because with Darla, with the lack of true completeness and love like he shared with Buffy, he doesn't experience the moment of true happiness that activates the clause. Instead, he gets an epiphany and slowly begins to work towards rebuilding everything he spent the last weeks steadfastly destroying; his life, his relationship with his friends, even his mission - he rejoins the AI team, but not as the boss, as an employee. He will reclaim his rightful position eventually, but not before he wins back the trust he's broken.
Months later, Darla returns again. But this time, she's pregnant…