Ok so you have to keep this in mind before you read this: SerRei and ClearAo (Clear x Aoba) have two different kinds of love. Clear is EXTREMELY devoted to Aoba from the very start. While SeriRei has a very simmered and understated affection for one another (at least that's how I see it). Their similarities lie elsewhere.

So Clear is a defective android from the mega cooperation Toue, Toue bought the island that Aoba lives in and made the living conditions shit by engulfing a third of the island with their project The Platnium Jail, a luxury resort. Toue also paid for research and experiments related to mind control. Aoba and his twin is a result of that, twins with altered DNA to have mind control abilities, Aoba controlling with his voice and his twin controlling with his eyes. Aoba was taken out of the research facility by his grandmother and his brother remained with Toue.

Clear is an android made with the artificial ability to control minds with his voice. But he was defective (as I mentioned), he was supposed to be destroyed but an old man had saved him and taken Clear to live with him in secret. Putting a gas mask on Clear, telling him never to take off his mask due to his face being different, even going the extra mile and covering the mirror in their residence with a blanket. Clear had remained hidden, until Aoba's voice brought him out. He stuck to him, mistaking him for his master Toue (the ceo named the company after himself).

When you go to his route, you discover the information about Clear I mentioned prior. Clear often mentions how much he's learning about life through Aoba, so when he confront Toue with the same questions he asks Aoba they don't match up at all. Toue sees life as a game to win, with everyone being pawns in this metaphorical game of chess. Seeing Clear as a disposable son. This very outlook angers Clear, causing him to defy Toue in an extreme manner by breaking the hardback he has which causes his body to deteriorate in order to sever his connection to him.

Where I feel this resembles SeriRei is the whole learning something new about life. And the rebellion against the person they're supposed to work for. Serizawa has been plucked while he was vulnerable and maluble. Molding him for his selfish goal of taking over the world (a similar goal to Toue's), Serizawa also took a huge risk rebelling against Toichiro by protecting Reigen before he gets vaporized. This makes Serizawa in Toichiro's eyes, defective. When Serizawa starts working under Reigen, he learns quite a bit from him. Like Clear, he learns a different outlook on life, a more positive one. They both cherish what they learn from their respective partners. It's a very surface level similarity. But it's a similarity nonetheless. My brain likes to connect my favs to one another.

Also both Clear and Serizawa carry umbrellas so that's a bonus.