The LSB Round Robins have already profiled SUPERBOY #89 (you can read our review here). So we aren't going to go over the particulars of their initial meeting here. However, there are two panels in particular we want to re-present for you:
Think on it: here are two of the strongest young men in the galaxy. Neither have any other siblings*, and then they happen to meet each other and hit it off immediately. Right away they consider each other "like brothers." One gives the other a "nick-name" that is embraced more fully than his given name. They are pals. (*relatives of Mon-El were later mentioned in Legion continuity, but he himself was never shown interacting with them.)
Then the unthinkable happens, and they have to part, possibly forever. It's a painful scene to watch.
For 1,000 years Mon-El is trapped in the Phantom Zone (only two years real time). Thankfully editor Mort Weisinger decided to move the character into the 30th Century and join the Legion. From there he was always a stalwart member of the group, serving with distinction and eventually being voted Leader. He often teamed up with Superboy, such as when they went to Smallville together to escape from Mordru, or as when they tried to go on a reconnaissance against the Time Trapper, as shown below. Remember the Time Trapper; he'll come up again later.
Because Mon-El first appeared in "present day" Smallville, he had ties to that city that were stronger than any other Legionnaire. In fact, he sometimes visited Smallville just to vacation there! As a time traveller he could in theory have gone back and visited his own parents, but we never saw that he did. Instead, he visited "his brother's" family, the Kents.
And eventually, "time" caught up to him. When the Legion decided to face off against the Time Trapper once and for all, Mon-El led the fight. They first went back to Smallville to meet up with Superboy. That's where we get these affectionate scenes:
Of course, Superboy ends up paying the ultimate price to save Smallville and his Legion friends. Mon-El is the first mourner at the passing of Kal-El. His death affects him deeply.
The only thing I think that could have made it better is if at the end Mon-El had whispered Superboy's real name to him....
Finally, Mon-El is one of the Legionnaires willing to give his own life to make sure that the Time Trapper never hurts any one else. He and three others take a vow to defeat the Time Trapper, even at the loss of their own lives. The Legion is able to defeat the Time Trapper, and, at peace with himself, Mon-El drifts away into a death-like coma.....off to play baseball with his younger brother again.