Cilan is one of three Gym Leaders operating the Striaton City Gym alongside his brothers, Chili and Cress. The gym also functions as a café managed by the trio. Specializing in Grass-type Pokémon, Cilan awards the Trio Badge to victorious challengers. His appearance features chartreuse green hair styled with a distinctive four-pointed tuft, green eyes, and a waiter-inspired outfit: a white shirt, black waistcoat, black pants, a white apron, brown shoes with golden buckles, and a green bowtie matching his hair and eye color.

He holds the classification of an A-class Pokémon Connoisseur, capable of evaluating the compatibility and bond between trainers and their Pokémon. Cilan frequently declares "It's evaluating time!" when performing assessments, adapting this phrase to various situations (e.g., "It's detective time!"). His personality is notably eccentric, cheerful, and dramatic. He demonstrates extensive knowledge across multiple fields, including Pokémon battles, culinary arts, fishing techniques, and subway systems—the latter earning him the self-proclaimed title of "Metro Connoisseur."

His journey begins when Ash Ketchum challenges the Striaton Gym. After losing the decisive battle to Ash's Oshawott despite a type advantage, Cilan is impressed by Ash's battling style and bond with Pikachu. He subsequently leaves the gym in his brothers' care to travel with Ash and Iris through Unova, aiming to broaden his experience as a Connoisseur. During this journey, he acts as the group's navigator and primary cook.

Throughout his travels, Cilan captures and bonds with three Pokémon:
- **Pansage**: His first partner, obtained during childhood. They share a deeply trusting relationship and often collaborate in tasks like brewing tea or devising battle strategies.
- **Dwebble**: Encountered while crafting its rock shell home. After helping it retrieve the shell from bullies using a plan involving Pokémon food bait, Dwebble willingly joins his team. It later evolves into Crustle during a battle against Team Rocket, with Cilan enthusiastically declaring "Evolution time has begun!"
- **Stunfisk**: Caught while fishing for a medicinal herb. Cilan confuses and incapacitates it using specialized rod techniques before capture. Though their bond is less explored, they demonstrate mutual care.

Cilan develops significant relationships:
- **Ash**: He respects Ash's passion and battle intuition, forming a close friendship where they support each other's growth. They later reunite briefly in Galar.
- **Iris**: He values her expertise with Dragon-types and occasionally offers her guidance, though his exuberance sometimes annoys her.
- **Rivals**: He incites a one-sided rivalry with Burgundy, a C-class Connoisseur he previously defeated in a gym battle. Despite her hostility, he remains encouraging toward her progress.
- **Family**: He maintains a supportive dynamic with his brothers, notably helping Chili refine his offensive battle style and teaching his Pansear Solar Beam.

Cilan faces personal challenges, including a deep-seated fear of Purrloin stemming from a childhood incident where one attacked him with Fury Swipes when he lacked a protective Pokémon. This fear resurfaces during a connoisseur duel but does not hinder his performance. He participates in several tournaments, including the Club Battle, Clubsplosion, and Junior Cup, achieving varied results. After traveling through the Decolore Islands, he parts ways with Ash to journey through Johto. He later appears in Kanto, collaborating with Brock to calm an injured Gyarados, and in Kalos, where he partners with Clemont for a research project.