Dear Big Sister in Heaven, My Niece Who Became the Hero is Too Erotic. Signed, Her Uncle. Being Something like Her Guardian Will Soon Be Impossible for Me+

Angel, a former top-tier mercenary, was forced to retire due to an injury. Left by his fiancée, he now spends his days drowning in alcohol—a middle-aged adventurer (and a virgin). One day, his niece Liz, whom he once taught swordsmanship, suddenly barges into his life!