knabino

Knabino is knabo plus -in-, the feminine suffix, and the same pattern gives patrino, filino, fratino and reĝino. That asymmetry, where the male form is the bare root and the female is derived from it, is the single most argued-about feature of the language. Reform proposals add a matching masculine suffix so that knabo becomes neutral and knabiĉo is specifically a boy, and you will meet that usage online. The classic system is what the corpus is written in, so learn it first and take a view later.