Does my chosen champion always have to match my legend?

To clarify your question, we need to distinguish between "chosen champion" legality for deck building and the rules regarding matching your Legend.

According to RiftJudge FAQ #3966 and Rule 103.2.a.2:

• Your Chosen Champion must share a character tag with your Champion Legend.
  • For example, if your Champion Legend is "Lux, Illuminator" (tag: Lux), you must choose a Champion Unit that also has the "Lux" tag (like "Lux, Dawnbringer" or "Lux, Star Guardian").
• You do NOT have to play the same specific card as your Legend.
  • The "Chosen Champion" is a specific slot in your deck construction. The unit you put there must share the character tag of your Legend, but it does not have to be the exact same card name.

Important Nuance: Per RiftJudge FAQ #6279, while your Chosen Champion (the one in the Champion Zone) must match your Legend's character tag, you are allowed to include other Champion units in your main deck that do not match your Legend's character tag, provided they fit within your deck's Domain Identity (the colors determined by your Legend).