I suppose I'd better start at the beginning, or at least, what I would assume was the beginning. Our first confirmed sighting of the Changeling came during the X-Men's showdown with the villainous Factor Three (I will bet all the money in my pockets that no other comic book story in history starts with the heroes taking odd jobs to earn plane tickets to Europe, and ends with them defeating an Sirian octopus before it can unleash a storm of thermonuclear weapons upon humanity). Changeling is the second in command of the aforementioned outfit, under the command of the aforementioned extra-terrestrial cephalopod, masquerading as"The Mutant
Master". Their stated goal: obliterate the human race by the simple expedient of irradiating the crap out of them, leaving the world free for mutants. Well, for the evil mutants, at least, Double M is very clear about how the world will be safe for evil mutants (seriously, how did anyone buy into a single thing this guy said?).
The X-Men arrive at Factor Three's hideout, in attempt to rescue the kidnapped Professor X. Instead, they are captured themselves, and put on trial for crimes against mutants ("crimes against mutants" being defined here as "not letting other mutants commit crimes"). Ultimately, the day is only saved when Changeling turns against his master by transforming into Professor X and tricking him into confessing that his ultimate goal is the frying of all humanity, regardless of the specifics of their DNA spirals.
That is the last time we see Changeling as himself. Three issues later, he is dead, not that we realise it at the time.
In previous articles I've mentioned alternate universe versions of our heroes, on the theory that even twisted mirror images can offer some insight into character. With Changeling, we don't really have much choice.
When a new X-Men cartoon was slapped together for the nineties, it was decided that a character would die in the first episode, to demonstrate how edgy and unpredictable and, well, nineties it was. They didn't want to kill off a character anyone might have heard of, of course (there's edgy, and then there's suicidal), so they based a new X-Man around Changeling, and called him Morph. Ultimately, the shape-changer proved so popular in his brief appearance that he returned for several episodes. This in turn caused Marvel to write the character (who apparently shared nothing beyond a name and the ability to shape-shift) into the Age of Apocalypse, most prominently in the four-issue Astonishing X-Men series. I'm not sure if it's ever been explicitly stated that the two characters are one and the same, but there are enough clues and head-nods to assume that this is the case.
Almost everything that can be surmised about the Changeling in our own universe comes from the two most prominent alternate-universe Morphs, the first from the Age of Apocalypse, the second from The Exiles comic. The most obvious characteristic the two share is a spectacularly immature sense of humour, combined with a total inability to recognise when to shut up. It's probably far from surprising that someone who can alter the shape and colour of his body into anything he wants to wouldn't feel the need to develop his humour to anything beyond obvious visual jokes, but of course the specifics of his jokes are far less interesting than why he bothers. In the two realities, he gives very different explanations. In the war-torn blood-drenched hell-hole of Apocalypse's Earth, he simply points out that he is determined to enjoy his life for every moment possible, until the very second an Infinite finds some way to kill a man made entirely of unstable molecules. When travelling with the Exiles, the second Morph admits that his refusal to take things seriously comes from rebelling against his severe father, who demanded Morph buckle down and control his life after the passing of the shape-shifter's mother.
Like the Joker in Batman Begins, there's no way to tell which explanation is true, or if neither are. It's possible both are true given their origins in separate realities. The link between the two, though, is that humour is clearly Morph's defence mechanism. The worse his life gets, the more likely he is to play the fool. What may have began as an act of defiance against his father, and certainly becomes an act of defiance against the wishes of his superiors (and anyone else nearby) becomes defiance against life itself. Only on occasion, when he is attempting to comfort the one most important to him (Blink or Mariko, depending on which Morph we're talking about) does the mask slip, and tellingly, it works all the more for being an occasional event. The rest of the time, even as he enrages Rogue in order to prevent her giving up in despair, or has Rachel Summers call out to the Gods of Asgard, it's done with an irritating smile and a terrible pun.
It's this mixture of defiance and the attempt at all costs to pretend he doesn't actually notice or care how things have become that informs Changeling in our own world. It isn't entirely clear at what point he decides to betray the Mutant Master, but he certainly did it without knowing the extent of the Mutant Master's plan. For all Changeling's crowing over his upcoming vice-dominance of Planet Earth, it is likely that he is already planning his rebellion.
Soon after the Mutant Master crisis (and by soon, I mean two issues at most), Changeling approaches Xavier to tell him that he has been diagnosed with terminal cancer. Rather than spend his remaining weeks continuing to live the life of a super-criminal, or transforming into Hugh Hefner and high-tailing it to the Playboy Mansion, Changeling resolves to go out fighting for the X-Men. Let the world decide that it doesn't want him in it. He'll spend his last days doing everything he can to help that shitty, pointless, uncaring world as much as possible before he punches his ticket. In the end, despite having started off as a would-be world conqueror, once life shafts him hard enough his default position is to become a hero. And yes, this is back in the day when ancillary characters leaped across the hero-villain line like borg-gazelles , but still worthy of some respect, I think.
Ultimately, Xavier makes use of him as a body-double, whilst unknown to the X-Men (excepting Jean) he hides in a psi-proof chamber and plans a defence against the alien Z'Nox. This state of affairs that lasts for at most three issues (I don't know whether or not the changeover occurred before the X-Men fought Frankenstein's Monster, and once again I am not remotely kidding) before Changeling sacrifices his life preventing the monster Grotesk from destroying the Earth. It is months before the X-Men discover the truth and years before we do. Changeling's only reward: a photograph alongside those of Thunderbird and Doug Ramsey on the Professor's desk, a reminder of the fallen.
Still, he did better than Petra or Sway, soon to be appearing in this series under the heading "Who the fuck are Petra and Sway?"
Next time, we consider the green-haired magnetism-wielding Polaris, and lament the fact that such a powerful female character ended up so mangled by various writers that there wasn't anything left but bitchy one-liners and the occasional attempt at mass-murder.
 Let's not forget the Mimic, who within his first four issues went from belligerent hood to boastful crime-fighter to flunky to a world-conquering super-villain to noble self-sacrificing hero. Final line: "Funny... It took an inhuman, emotionless thing like the Super-Adaptoid... to make me realise the true value of the emotion called... friendship! Even if the Mimic is gone forever it was worth it if Cal Rankin became... a man."