Who are some important characters never seen but always mentioned?

The original post: /r/television by /u/KomturAdrian on 2024-06-20 02:44:16.

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I was watching Seinfeld and remembered Kramer has a friend named Bob Sacamano. He is always mentioned but never makes an appearance.

Are there any tv shows that have characters like that - named and mentioned but never seen - who are very important or who influence the plot?

Even characters who speak but are never seen also counts, like Charlie from Charlie’s Angels.