呢 = 口 (mouth) + 尼 (ní, for the sound). Like 吗, it's a spoken particle marked by the mouth radical, toneless: ne.
呢 has two friendly jobs. First, it bounces a question back: someone asks 你好吗?, you answer and add 你呢? — 'and you?'. Just 我呢? means 'what about me?'. Second, it softens a question about where/what: 我的书呢? ('Where's my book?'). It makes speech feel warm and conversational, not blunt.
A mouth (口) particle that lobs the question back: '…and you?'
呢 is the conversational ball-return: answer a question, then add 你呢? to send it back. 我很好，你呢? ('I'm well, and you?') — instant natural small talk.