Butt Monkey: He really hates Legion try-outs, usually because he winds up on the receiving end of someone's demonstration.
In the spin-off comic of the cartoon, during Infectious Lass's try-out, he wound up with a form of lava acne and got stuck with Bouncing Boy's power (and physique). What sucked worse is that, the virus that switched his power with BB's lasts longer on people with greater mass.
Cynicism Catalyst: The death of his sister Ayla during the attack that granted him and his older brother Mekt their powers pushes them into their guilt-ridden Cain and Abel complex, along with Garth's cynical attitude and Mekt's path into villainy. Subverted in that she didn't die at all and was eventually restored to a physical form.
Good Parents: He was seen embracing them after the storm that devastated Winath, happily embracing their children in the photo before he and his siblings got their abilities and him bringing Ayla home to see their parents.
Real Men Wear Pink: Has surprisingly good bedside manner, as demonstrated by him trying to get Matter-Eater Lad to eat so he would recover quickly in the hospital. And then of course there's the fact that he loves his little, formerly twin sister dearly.
Teeth-Clenched Teamwork: With Dream Girl in season two. His reason for doing so is that she used to be a con artist fortune teller called Madame Mysterious, but the real reason behind his dislike is she told his parents that they would find his sister Ayla on another planet and it never happened, which devastated the two. Though their relationship has softened by the end of Nura's spotlight episode.
Genius Bruiser The genius part is blatantly obvious, the bruiser part comes in when he turns into his "battle mode"
Read: 12 foot tall mecha that has enough dakka to make someone supposedly "armed to the teeth" and member of the bad guys who took the whole legion to run like a little girl
I Did What I Had to Do: Less of a single occurrence and more of a defining motivational trait - Brainiac 5 often makes the tough, pragmatic and/or rational decisions, and willing to pay high costs (including sacrificing himself, or on rare occasions others) if he feels it needs to be done or the alternative is something much worse. This extends to everything from saving the day to preserving the timeline, and becomes particularly obvious in "Message In A Bottle" and other episodes dealing with Brainy ensuring that Superman doesn't learn too much about his past.
Pointed out by the original Brainiac, who uses this part of Brainiac 5 to create a connection between himself and Brainy. As his resistance breaks down, Brainy slowly starts to believe that his tendency for this makes him Not So Different than Brainiac 1... and that's all 1.0 needed.
Resist The Beast: In "Cry Wolf" he does experience this trope and trying hard not to revert into his bestial form in one such instance he was struggling with his monster form when Phantom Girl tries to coax him back to normal...until the Science Police came in and attacked him causing him to run and fly to Rawl so that he wouldn't be a danger to anyone else.
Vomit Discretion Shot: In the spin-off comic of the cartoon Timber Wolf gets motion sickness on the trip back from Rawl, fortunately Superman manages to get him off the ship before he could show the Legion what he had for lunch.
Literal Split Personality: Each of the three duplicates she splits into are semi-individuals with slight differences in personality to the point that they're both one person and yet also like very close sisters.
Genetic Engineering is the New Nuke: Created specifically to turn the tide against Imperiex in the 41st Century, he's the result of Superman's DNA being mixed with Kryptonite and as such is immune to the green rocks effects. It also has the added side effect of granting him different powers.
Unresolved Sexual Tension: With Saturn Girl. In "Chain of Command", he tried to protect her from harm's way, but she told him he can take care of herself and leaves before he can apologize. In the episode "In the Beginning", he's upset with Saturn Girl for always taking Lightning Lad's side.
What the Hell, Hero?: Lightning Lad calls him out for not even having the time to visit Saturn Girl while she's in a temporary coma.
In Name Only: Comicbook Imperiex was a present-day Energy Being in a suit of armor, so giant that most of the time you were Fighting a Shadow; Animated Imperiex is a human-sized alien cyborg from 41st century.
Both have titles in front of their names, related to their professions and are referred to as such (President Wazzo and Dr. Londo)
They are single parents to their children (who're the same gender as their respective parents)
Their children are in the Legion of Superheroes and are close friends.
Their children resemble them in some way (for example, take away the werewolfism and his son has his facial features and hair. In a few glimpses of his son's human form he does look like a younger version of his father)
They both meddle in their children's lives. The key difference is Winema is well-meaning and ultimately wants what's best for her daughter while Londo only is interested with what he wants and doesn't care what happens to his son, he just wants his weapon/general.
From Nobody to Nightmare: In the original comic series he was only used for the purpose of flashbacks with little if any character development but when the animated series came along he may be as bad of a parent as Ozai
Jumping Off the Slippery Slope: At first appears as a lone, grieving scientist who's lost everything to the creature in the forest. As it turns out his son isn't dead but is the creature Londo told the Legion about he attempts to kill/capture the Legion and take his son back by force using his animal test subjects.
Karma Houdini: One of the very few villains of the series not to be either imprisoned on Takron Galtos, sent to the Phantom Zone or killed