What role does a shield play?

[PvK]

Yes the early main weapon was the bow, not the katana, and heavy use of bows continued, perhaps moreso than in many western ancient/medieval armies. Late medieval Japanese armor does seem to have been relatively effective against arrows, judging from the number of arrows frequently shown sticking out of still-fighting samurai in art (and samurai films such as the Last scene of Throne of Blood). Post-gunpowder, some breastplates were tested to be sure they were proof against muskets, though I think they still were only a part of the torso coverage, with plenty of places still to get hurt.

Later samurai equipped for battle would also tend to carry a naginata as the first weapon, which is a two-hander.

Although they did have some shields, perhaps like their sword style, they tended to emphasize avoiding arrow hits, rather than blocking them.

PvK