Namely, the cheery character and the gloomy character.

I'm looking for a manga with this sort of plot:

• At the beginning of the story, Person A is happy-go-lucky and optimistic, while Person B is grumpy and aloof.

• Throughout the story, Person A attempts to make person B see the beauty of life, or at least make them less grumpy.

• Person B usually rejects Person A's attempts, but Person A still tries anyway.

• An incident or some sort of traumatic event happens to Person A, making them a bit more cynical and less cheery than before.

• Person B now tries to bring back Person A's old self back by attempting to cheer them up, the same way Person A did.

In short, it's pretty much a role-reversal between the two characters.

It doesn't necessarily have to be a romance, though it would be better if it were.

I wouldn't want:

• Yaoi/Yuri
• Shonen-ai/Shoujo-ai
• Hentai
• Mecha

I normally don't read, but I might consider only if the story is THAT good:

• Gender-Bender
• Smut
• Sports

Thanks again!

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Maybe One Week Friends Not exactly what you're looking for, but in the most recent chapters the main girl who was reclusive is getting more out-going and cheerful, while the main guy who was happy-smiley is getting sad... The guy did help the girl come out of her shell, so that part fits.
Note: Some of the middle pages/sections of the story are in 4-koma format though

M.A.X something like that happens near the end.
Berserk seinen, I'm not sure if this is to your tastes, but something like personality change does happen later on.
The Moment When a Fox Becomes a Wolf does have role reversal later on.
Hatenkou Yuugi this event happens much later in the series, but the personality change in the male MC becomes more apparent early on.

I vaguely remember reading something that matches your description, but I still can't grasp the title. Will update if something comes to mind.

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My syntax sucks, I know.