Sui is a slime first encountered as a baby during Tsuyoshi Mukouda's travels. Defying typical weak slime stereotypes, Sui immediately exhibits unusual intelligence and a powerful attraction to the smell of Mukouda's cooking. After being fed, Sui voluntarily forms a familiar contract, becoming Mukouda's second contracted creature. Sui undergoes significant physical and combat evolution. It gains strength and abilities by consuming magical items and trash from Mukouda's Online Supermarket purchases, alongside battle experience. Starting with basic acid attacks, it advances to firing precise "acid bullets," often targeting enemy heads accurately. Sui further evolves to produce high-grade potions with exceptional healing properties, unseen in ordinary slimes. Eventually, it acquires advanced crafting skills, creating weapons and tools like an "absurdly sharp" mithril blade and a meat grinder at Mukouda's request. Sui exhibits childlike enthusiasm and curiosity. It communicates telepathically with Mukouda and other familiars, showing excitement for new experiences and foods, especially desserts like custard pudding. Sui displays clear preferences, disliking spicy items, and forms affectionate bonds, referring to the Fenrir Fel as "Uncle Fel" despite Fel's pride. Its relationship with the water goddess Rusalka develops into a sisterly dynamic. Within the adventuring party, Sui becomes a key combatant despite its appearance. It actively battles formidable foes like wyverns and dungeon bosses, contributing to the group's reputation as the "Invincible Trio" alongside Fel and Dora-chan. Sui's telepathic coordination with Mukouda exceeds standard tamer-familiar bonds, attributed to their voluntary contract. The goddess Rusalka grants Sui her blessing, enhancing its water-based magic in exchange for offerings of otherworldly cuisine. The spin-off manga "Sui's Great Adventure" details Sui's perspective and growth. It explores Sui forming friendships with humans and cats in Karelina city, experiencing festivals like "Reunion Day," and navigating emotional challenges such as farewells when leaving. These stories highlight Sui's increasing social integration and emotional maturity encountering diverse situations and characters, including a deepening bond with Rusalka. Sui's status as unique is formally recognized when the god Demiurge confirms its extraordinary nature compared to ordinary slimes during a significant blessing bestowal.