I don't remember who asked for it, but someone was curious about Keith's feelings about Lance in relation to Shiro.
Lance is a straight omega (attracted exclusively to alphas), which Shiro is a prime example of.
Keith really wished Lance was an alpha, then he could just deck him without being the bad guy. He didn't understand why the omega was constantly needling him, and the way he always stood so close to Shiro, with those big shiny hero worship eyes. It didn't matter how much he scent marked the man, Lance never seemed to catch the hint that Keith didn't appreciate him getting all up in his mate's personal space.
It was like being back at the Garrison, where it seemed like every other omega viewed him as just keeping the spot warm. He'd wanted to punch them too, and when a female alpha asked Shiro to go somewhere private right in front of him he'd hit her. She'd at least had the decency to claim nothing happened when they were questioned afterward.
If he was fair, and he really didn't want to be, Lance had never made a pass at Shiro. He'd flirted with everything that even vaguely resembled an alpha on up to the Princess, since they'd gotten into space, but he'd kept his pick up lines away from the Black Paladin. He just got too close, and made those damn eyes at him.
It really shouldn't have surprised anyone when it came to a head.
The mission had been long and life threatening, when Keith got out of his lion all he wants to do is grab Shiro and go to sleep. What he gets, is walking into the hall to find Lance hugging on his mate. Shiro isn't hugging him back, even as Lance is getting his omega scent all over the alpha. Keith snaps.
Lance nearly hits the floor as Keith tears him off Shiro and shoves him away.
"What is your problem!" Lance shouts.
"You, you are my problem!" Keith yells back, then he makes a gesture to emphasize the space around him, "This is what we call a personal bubble, Lance! Learn to freaking respect it."
A big hand rests on his shoulder, "Keith it was just a hug," Shiro says, trying to defuse the oncoming fight with the tone of his voice alone.
Keith's having none of it. He whirls on Shiro, "Does he know that!" He shouts, pointing at Lance, "Cause he sure seems keen on ignoring the fact you're taken!"
"Dude, I am not trying to steal your mate!" Lance yells, interjecting himself back into the argument.
Keith turns and stalks towards the omega. Lance takes a step back, eyes going wide, "Really? Could have fooled me." Keith snarls, he follows as Lance tries to backtrack, pushing up close to get in his face, "Does this make you feel uncomfortable? Like I'm a little too intimately close? Maybe it's because I am! And you shouldn't be getting this close to other people's mates!" He shoves, and this time Lance hits the ground.
"KEITH! Back off Lance." Shiro shouts, and suddenly his Galra hand is wrapped around Keith's bicep. "You. Go now," Shiro says to Lance, pointing towards the door. The omega scrambles to the obey, pushing off the ground, and running for the door.
Keith waits until Lance is gone to yank his arm out of Shiro's grasp, "Going to try to pretend I'm in the wrong," It'd be typical of his life. He gets pushed and pushed, until he finally pushes back, then everyone acts like he's the one with the problem.
"You're overreacting," Shiro says sternly, Garrison approved look of disappointment on his face.
"You're underreacting!" Keith snaps back, "You're always underreacting. Letting every Tom, Diane, and Harry hang all over you." Was it any wonder that everyone had always thought he was just a place warmer, when Shiro let absolutely anyone that wanted to get close to him.
"I'm not going to cheat on you," Shiro says.
"I don't think you're going to cheat on me!" Keith shouts again, it's just so frustrating. Shiro is completely missing the point, "I just want some respect." He emphasizes, "I want people to stop acting like I'm not here and that you're available. I want you to stop encouraging it." Stop acting like it wasn't happening, "I want you to put up some boundaries, so people will stop hitting on you."
"Keith," Shiro's voice has softened. The older alpha reaches out, tries to pull Keith in.
Keith bats his hands away, steps back out of reach, "Don't try to hug me. This isn't a hug and make it better type of situation." He growls.
"I'm sorry," Shiro has his sad puppy dog eyes on, and Keith has to look away. No. This is a real problem and he won't let Shiro smooth it over with hugs and kisses.
"No, you're not. You're not going to do anything about so you're not." He says it to remind himself as much as Shiro. The universe will cease to exist before Shiro's personal space stops being open to all who seek it.
Shiro tries to say something, but Keith cuts him off, "Don't. Just go to bed Shirogane." Maybe the cold sheets will get him to think about what Keith said, "I'm going to train. I'll see you again, when I don't feel like ripping your damn head off." Keith storms out.
Note:
A/B/O universe, violently defending your claim means something different than in our own.
Should be clarified (because I got asked this a lot on other sites), in this universe Lance crossed the boundaries of good tastes. If he'd been behaving that way to another omega and their mate, a fight would have started much sooner. Not a bad guy, just a bit boorish.