Note: If you are looking ahead in the Siege of Terra series, finding reviews of potential books to read, strongly consider reading Garro: Knight of Grey before Mortis. It is listed after Mortis due to publishing date, but exists prior in the internal chronology of the series.
In my lurking around Warhammer forums and Horus Heresy discussion, it seems Garro, the Death-Guard-warrior-turned-warning-beacon in The Flight of the Eisenstein, has clout among some readers. People seem to like him. He doesn't stand tall in my reading of 30k, however. He has been more of a flat, hardboiled stereotype not to mention incidental character—one who happened to be in the right place at the right time—rather than a character with proper agency worth developing in the series. Take this review of Garro: Knight in Grey (2022) with that in mind.