At the end, I still hate that Ruri got actually traumatized by being shoved over in primary school. Most kids have at least a brawl around that time, along with bullying.
Also, I found it very funny that Seri's tough love with Shizuku in the last chapter helped Shizuku a great deal in the end, when her coddling of Ruri is very likely a big reason Ruri is still stuck in the past.
One could argue that it's a bit over-the-top, but it's the incident the two of them remember most- for Ruri, it was the first time Shizuku laid a hand on her, and for Shizuku, it was when she was confronted with the reality that she was bullying Ruri..
It also effectively sums up Shizuku's treatment of Ruri- if you look closely, Shizuku appeared to just give Ruri a light slap on the back, only for her to actually fall over, and Shizuku's expression was clearly surprised, horrified and/or guilty as that happened (even if she brushed it off when Seri confronted her).
That's an interesting take on Seri. She was also the first person who explicitly called Shizuku's treatment of Ruri bullying, hence Shizuku's shock; even the people who complained about Shizuku blaming Ruri for their loss in the volleyball game didn't use that word. One could also use that reasoning to conclude that while Seri was right that meeting Shizuku again was painful in the short term, in the long term, it actually helped her see how much Shizuku had changed and get her to take steps to change herself..