Ikkyū-san appears as a minor character in "Zan Sayonara Zetsubō Sensei," presented as a childhood acquaintance of the protagonist Nozomu Itoshiki. He initially claims to share Nozomu's fondness for antiquated objects and styles, suggesting a nostalgic connection. However, it is clarified that their relationship was not a long-term friendship but a brief, transient interaction during elementary school. This distinction hinges on wordplay: while the name "Ikkyū" phonetically suggests "old friend" (旧友), it is revealed to mean "one-day friend" (1日友), emphasizing the fleeting nature of their past association. The character's name references the historical monk Ikkyū Sōjun, popularized by a separate 1975 anime series depicting him as a witty problem-solving child monk. This connection serves solely as inspiration for the name and does not inform Ikkyū-san's portrayal or narrative role in "Zan Sayonara Zetsubō Sensei." No further details about his background, personality, development, role beyond elementary school, or appearances in other seasons, films, OVAs, or spin-offs are provided.