kusukes lab

Obviously this all starts when Kusuo is born. Pretty normal sibling rivalry for a while, though it escalates as they both get older and stronger and their parents don't stop them.

Kusuke creates the limiters. One limiter was supposed to stop Saiki's powers while inserted, and the other one was to keep Kusuo hostage for their games. I believe Kusuke thought that by suppressing Kusuo's powers, he could establish the "proper" hierarchy of older brother beating younger brother. But it wouldn't be a complete win for him without beating Saiki at full power, so the second limiter ensures Saiki will still compete with him. Kusuke's trying to get better at games and fighting, so he's thinking that if he can beat Kusuo with the limiter in, then he just has to train until he can beat Kusuo with the limiter out.

Up till this point, there's no evidence that there was any sexual component going on. And conveniently, right as he would be hitting puberty, Kusuke high-tailed it to Cambridge. At this time he still thought that by working with other scientists, they could figure out how to completely remove Kusuo's powers.

After that, Kusuke doesn't make an appearance until Kusuo goes to him about the broken limiter. He's left cameras all over the Saiki house which is bizarre and wrong, but to be slightly fair to him, he grew up without any privacy due to Telepathy, so it kinda makes sense. And technically if something went wrong with Kusuo, Kusuke is the only person who could help, so it would make sense that he should keep an eye on things. Still fucked, but he's not exactly an ethical guy overall.

Once Kusuo needs his help, Kusuke tests to see if he can finally beat Kusuo conclusively. He hasn't done this since he left Japan, and it's actually entirely possible that this is the first time that he ever reacted like that to a loss. Still, despite apparently having the best time of his life, he's annoyed enough by the next morning to bully Nendo and Kaido for no reason. Perhaps he realized that his excitement was messed up??

After this, Kusuke doesn't show up again for a while, except to warn Kusuo and their parents that there was a defective part in the limiter. The fact that Kusuo accidentally screwed up the past and met the WWIII AU Dr. Kusuke isn't technically Kusuke's fault and was definitely not his intention.

Almost every single Kusuke appearance after this point is directly related to The Final Game, in which Kusuke will defeat Kusuo in a super epic battle and then cement his dominance forever (and destroy the rivalry / humiliation kink potential) by slam-dunking the power deleter into Kusuo's brain.

Kusuke's next appearance is scamming his grandpa. Not very nice, but I think he was actually testing how much his grandpa loves him before moving in with his grandparents and launching his "elderly robot gang" plan of attack on Kusuo. Notice that his grandmother, who Kusuke likes better and brings gifts for, doesn't get put in a robot suit later on, she gets eternal youth like she wanted (well, at least Kusuke's best shot at it). But Grandpa was definitely going to be used against Kusuo as a hostage and to Kusuke it's justified because his Grandpa doesn't love him.

The elderly robot gang plan is revealed and fails, so Kusuke has to think of some other way to defeat Kusuo in a super cool matter.

In the next new year's chapter, both Kusuke and Teruhashi show up at the Saiki household. It's not clear why Kusuke showed up (perhaps he just wanted to hang out with his family and mess with Kuniharu), but he does seem to test whether Teruhashi could be good hostage material - the grandpa plan failed and we know he later recruits Toritsuka. Luckily for Teruhashi, she's able to prove that she is NOT to be messed with, so that's another angle that's been shut down.

Next Kusuke sends the birthday bomb trials. This is explicitly to measure Kusuo's strength as Kusuke comes up with the perfect counter weapons. He's not trying to kill Kusuo after all, and later he actually gets nervous that the cat tank blast might be too powerful.

Then there's the KochiKame parody chapter, where Kusuke has a little fun getting Kuniharu his job back. No attempts to mess with Kusuo in this one and they interact super normally.

Then Warp the robot cat debuts. Since roboticizing humans didn't work for him, it seems Kusuke is going all-in on developing robots with as much functionality stuffed into them as possible. While Warp also serves as a gift for Kurumi, I think its also a step towards developing Kusuomega.

Ok there's also a chapter where Kusuke tries to get his grandma to divorce his grandpa. Just Kusuke being an asshole lol.

Kusuomega appears!! Since Kusuke's original plan was to throw a bunch of robot old people at Kusuo, it makes sense to test whether throwing a bunch of normal robots at Kusuo would work. Yeah, making Kusuomega lick his shoes is fucking bizarre and making him "fully featured" is also messed up. But Kusuke does also have to prove that he's willing and able to make Kusuomega do freaky shit at Kusuo's school to "properly motivate" Kusuo to fight back at full power. Kusuke puts a lot of effort into ensuring Kusuo doesn't half-ass their games and fights, going all the way back to the installation of the limiters.

Kusuomega was easily defeated, but instead of making more robots, Kusuke realizes he can use Toritsuka as both a weapon and a hostage. He immediately gets to work on that.

Then the cat tank arc, The Final Game. Kusuke was going to win Once And For All.

Except he didn't. Still, he gives Kusuo the option of whether to use the power deleter or not, without the leverage of publicizing his powers. Kusuke has accepted that he can't beat Kusuo, and that he's never going to get another chance to beat him full strength, because he knows Kusuo is going to use that power deleter. This is the ultimate sacrifice for Kusuke, since Kusuo's powers are his one interest and passion in life.

From then on, Kusuke is completely cooperative, helping Kusuo with the volcano problem and giving him genuine advice in trying to prevent his powers from returning. Naturally he gets a little excited at the prospect that Kusuo's powers might be back, but he does respect Kusuo's attempts to deny them back out of existence, setting up Kuniharu to fight the meteor instead of insisting Kusuo do it, even if this might mean they all die.

So in summary, it seems like:

Normal sibling rivalry -> limiters to try to end sibling rivalry -> escape to England -> Kusuo shows up so Kusuke tries to beat him for the first time in years and gets off on the loss -> Kusuke works towards a final conclusive game where he can win and then delete Kusuo's powers

While Kusuke justifies it as older siblings should always be superior to younger siblings, he clearly senses there's something wrong about their dynamic and he works hard to "fix" it in the only way he knows how: with an overwhelming win. And when even his biggest efforts fail, he accepts that he'll never win this way and lets go of their rivalry, shifting into a more normal sibling relationship. And Kusuo recognizes that and even seeks his help with the volcano for the first time.

So while their situation is definitely fucked for most of the series and Kusuke definitely did pretty much everything wrong... Things aren't QUITE as bad as they seem and it looks like the "bro-con" issue is gone by the end of canon.

Which sets Kusuke pretty far from Makoto, imo, who never recognizes for a second that his attraction is problematic and that even if that wasn't his sister, he should never treat another person that way.