Why these characters matter
The anchors worth learning first are the ones with the fewest lookalikes. Tensura has 5 characters, all listed for 2018, where red hair appears on only 1 of them, while blonde covers 1 — so a distinctive hair colour is often enough to locate the right cluster.
Learn Rimuru Tempest first, then the candidate it is most often confused with: Milim Nava and Shuna agree on 4 of the five clue traits. That one comparison removes the most common failure in Tensura mode.