So some spoiler chat was going on in the discord #LBspoilers channel and we were discussing how Rin didn’t had as much experience or exposure to the hardships in the routes as Riki did.
The way i see it Riki and Rin’s character developments are completely different. Riki is supposed to become someone who can support a person in their dire time (Rin) rather then only himself, While Rin’s supposed to become more social able and be more confident about interacting to other people.
Their trials for this were also different. For Riki, he helped komari see the sadness in the world, he supported her when she was trapped in that nightmare state and helped her get out of it. Same with haruka where he stood beside her providing comfort when she was dealing with the familys and Kanata and with kud he pushed her towards the choice so that she doesn’t regret leaving her mother.
Rin’s personality was dealt by komari and other busters. Komari first came and eased her fear of strangers by becoming her first friend outside of little busters. With her Rin came out of her shy shell enough to interact with other new busters, she became more confident with interacting with other people and we see that in progressive routes.
I think Kyousuke ment Riki to become stronger because he wanted him to support Rin when she (possibly) overcomes with despair from everyone’s death. He focused on Riki because he knew he can become the one who Rin can rely on from now on. It was necessary for Riki to face all those tragedies so that he can be that person.
Basically I’m saying that kyousuke put more faith in Riki and let him face the brunt of bat because Riki could handle it. While Rin couldn’t do that, i think overcoming her fear of people and becoming a Self-reliant person at the same time was too much for her. Maybe kyousuke thought that too so he focused that aspect on riki and only concentrated on making Rin more confident (that he also failed in because he rushed).