My all-time favorite legionnaire in DC’s Legion of Super-Heroes has always been Mon-El, especially as drawn by artist Dave Cockrum pre-X-Men. Of course, nowadays we know him better as Lar Gand, but his story across the comics ages (and remember, some of this is not canon since the coming of the New 52) has been an exciting one.
A precursor to the Lar Gand character appeared in the story "Superman's Big Brother", in Superman #80 (January–February 1953). He was named Halk Kar, and had a logo-less costume almost identical to Superman's, but with the red and blue colors reversed. He was created by Otto Binder and artist Al Plastino.
In that story, Halk Kar crash-lands on Earth in a rocket ship and is rescued by Superman, who discovers that Halk Kar suffers from amnesia. Discovering that Halk Kar has a note from Jor-El (Superman's father) mentioning his son, Superman assumes that Halk Kar must not only be from Krypton but he must be a son of Jor-El and thus also his own older brother.
Superman quickly realizes that Halk Kar is less powerful than he is and — instead of subjecting him to embarrassment over the fact that he may be weaker than his younger brother — opts to use his own powers to cover for Halk Kar's deficiencies. This plan backfires, as Halk Kar begins to assume a superior attitude to Superman and even begins to make romantic advances on Superman's girlfriend, Lois Lane. (Brotherly love?)
Finally, Halk Kar recovers his memory, and explains that he is from the planet Thoron, which is in the same star system as Krypton. Years ago, while on a pioneer voyage into space, he landed on Krypton with his damaged rocket ship. There he met Jor-El, who explained that Krypton's destruction was imminent and repaired Halk Kar's rocket ship, sending him away with the note which had a map from Krypton to Earth on it. Krypton exploded shortly afterward, causing Halk Kar to be put into suspended animation until he drifted to Earth to meet Superman, the grown-up son of Jor-El referred to in the note. Halk Kar returns to Thoron in his repaired rocket ship, leaving Superman with the experience of briefly having had a brother.
The Halk Kar plot was reused in Superboy #89 (June 1961), in a story set during Superman's career as a boy, one which simply disregarded the history outlined in the original (a practice common at the time). The character's name was changed to Lar Gand, his homeworld was changed to Daxam, and he was made younger to roughly match Superboy's age. He was an explorer who had landed on Krypton, where Jor-El warned him of the planet's imminent destruction and gave him a map to Earth. He suffered amnesia upon landing on Earth, where he met Superboy. As he gained powers like Superboy, the hero concluded that he was a long-lost brother, and named him Mon-El: "Mon" because he landed on Earth on a Monday, and "El" for Superboy's own Kryptonian family name. He adopted a human secret ID (Bob Cobb) to integrate into Superboy's hometown. (With time travel possible, maybe Halk was the hack?)
However the character was then shown to be immune to kryptonite, which is radioactively poisonous to all Kryptonians. Believing that Mon-El has been deceiving him, Superboy tries to trick him with a fake kryptonite meteor made of lead, which ironically turns out to be Mon-El's one weakness. Furthermore, exposure to lead is irreversibly fatal to Daxamites, which Mon-El explains, having regained his memory. Guilt-ridden over inadvertently poisoning him, Superboy saves his life by sending him to the extradimensional Phantom Zone, where he would be able to observe things happening in the outside world, but as a phantom would not age and his lead poisoning would not progress.
This provided for a means to use the character in contemporary stories set in the 30th century with the Legion of Super-Heroes. In these stories, Legion member Saturn Girl creates a temporary antidote to his lead poisoning, allowing him to be released from the Phantom Zone for brief periods of time, and he is considered an honorary Legionnaire. Brainiac 5 later creates a permanent antidote, and he becomes a full member. During his long career, he is written as one of the Legion's three most-powerful members along with Superboy and Ultra Boy, and serves two terms as leader. Mon-El apparently dies after failing to take a dose of Brainiac 5's anti-lead serum in a timely fashion. Eltro Gand, a distant descendant of his older brother, sacrificed his life force to restore Mon-El to life. He marries fellow Legionnaire Shadow Lass, with whom he has a long romantic relationship.
Through crises on several Earths, reboots and relaunches, and even a turn as a member of the Green Lantern Corps, Mon-El’s history has changed but the character has remained basically true to itself.
In today’s New 52, Mon-El eventually resigns from the GL Corps and returns to the Legion in order to become their field leader. It is further revealed that this version, as the entire Legion, is possibly the original one from before the Flashpoint event as they are even aware of these altering reality events.
Maybe one of the legionnaires can explain all the crises and altering realities to me!