Weirdly, we have "How I Met Your Mother" to thank for Mike Ehrmantraut’s existence. "Breaking Bad‘s" Season 2 finale “ABQ” was originally supposed to feature Bob Odenkirk’s Saul Goodman, but Odenkirk was busy filming HIMYM, so the writers created a new character to help clean up after Jane’s death.

The gruff underworld fixer was supposed to be a one-off, but Jonathan Banks made such a big impression as Mike, he came back for Season 3 and became a key part of the "Breaking Bad" ensemble, even reteaming with Odenkirk for six seasons of the spin-off "Better Call Saul".

https://tvline.com/lists/best-tv-characters-late-cast-additions-seinfeld-buffy-the-office/best-tv-characters-late-cast-additions-mike-ehrmantraut-breaking-bad/