AtD translation: the irregular of spirit

[Mike Jing]

P679.21-28   The moving field came to rest at length on whom but the
co-tenant of Tarot card XV, Professor P. Jotham Renfrew, apparently down
from Cambridge taking in a show, his face flattened into a lurid
two-dimensional chromo of itself,  sitting in a box with somebody in a
foreign uniform, whom it took Lew only a moment more to recognize as his
former fellow Archduke-minder, the Trabant Captain, now regular K. & K.
Landwehr Colonel, Max Khäutsch,  hardly changed from the Chicago days,
unless perhaps grown slightly more mineral, toward the condition of a
statue in a park frequented by the irregular of spirit.

What does "the irregular of spirit" mean here?