the fool is either the first or the last card in the tarot deck, being numbered 0; regardless, it is one of the major arcana, and generally the most iconic card of tarot.
this card can carry a positive meaning, as one of setting off to a new life and beginnings, shedding old habits. the knapsack here contains a tiger, which (besides just being cute) can represent the way atsushi is bringing only the tiger with him to his new life with the detective agency. atsushi's past is something he confronts a lot , so casting him as the fool is a choice that probably instead refers to the fool's title as the "protagonist"; often the major arcana is dubbed "the fool's journey", a person's journey through life.
(part 2 of the chara-ani tarot motif stands)