ŝuldanto

A participial nominal, and one of the clearest. Ŝuldi is to owe, learned at rank 902. -ant- is the present active participle, the one that means currently doing it. Put a noun ending on the result and you have a person defined by the action they are performing: ŝuldanto, one who is owing, a debtor. English builds its word the same way but out of Latin, and hides the seam so well that most speakers could not tell you that debtor, debt and debit are the same word three times.

The whole class works like this, and it is worth learning as a class. A parolanto is a speaker, leganto a reader, komencanto a beginner, loĝanto an inhabitant, vojaĝanto a traveller. The tense of the participle is live, not decorative: a ŝuldanto owes money now, a ŝuldinto used to owe and has paid, and a ŝuldonto is about to. No other language on this site lets you make that distinction in one letter. Participial nominals sets out the full system.

Mind the direction of the arrow. Ŝuldanto is the one who owes; the one who is owed is the kreditoro, and the noun for what is owed is ŝuldo, a debt. Register matters here too - this is a word from the world of the banko, the kredito at rank 2,387, the contract and the court. In ordinary speech you say li ŝuldas al mi kvin eŭrojn rather than calling your friend a ŝuldanto, in exactly the way English says he owes me a fiver rather than he is my debtor.

One historical note. Ŝuldo covers both senses of German Schuld, debt and guilt, and the Lord's Prayer in Esperanto uses it for both at once: pardonu al ni niajn ŝuldojn. The overlap is not an accident of translation. It is what the root has always meant.