Sigmarxism

if you want the uncertain loyalty chapter who might be renegade pretending to be loyalist or have entire other agenda not aligning with any faction Dark Angels successor chapter is probably the best choice (there's also Alpha Legion though DA successors can be more open), since the notrious secrecy of them all and all the Unforgiven nonsense, while it can provide somewhat of a protection against the Inquisition the minor chapters don't have.

Or maybe they thought Lion El’Johnson was a real lion

Lmao that would be great moment since he just returned, and is as grumpy as ever, as one of my friends said "a hipster grandpa".

The more important thing is that they have catgirls and catboys

Obligatory reminder:

Felinids (Homo sapiens hirsutus) - Felinids are endemic to the Imperial world of Carlos McConell. Whether this is due to the particular biological requirements of that planet, prior genetic modifications made to a Human population during the Dark Age of Technology or prior Imperial regulation is unknown, but they are virtually unheard of in the Imperium at large. Felinids are Humans who have had their genomes spliced with genetic sequences derived from various species of Terran felines, and they possess many felinoid traits, including a light covering of fur over their entire body, extended canine teeth and superhuman agility and grace.