My Ex-Girlfriend's Little Brother Is So Cute

After six years of dating, Takuya gets dumped by his childhood-best-friend-turned-girlfriend. Takuya has sunk to his lowest -- and then a message comes in from Naoki, his ex-girlfriend’s little brother. Naoki, who has grown into a stunning, long-haired young man, shocks Takuya with words that just might turn his world upside-down... A pure love story, featuring the never-endingly adorable Naoki, that is sure to make your heart beat out of your chest!!

(Source: MangaPlaza)

Notes:
- Originally a web manga published on the author's Twitter and pixiv accounts since December 27, 2018 under the title “Motokano no Otouto” (元カノの弟). Official serialization started in Comic Trail on December 6, 2019.
- Includes 8 extra chapters.