Samurai No Shield
It wasn t unheard of but just very uncommon.
Samurai no shield. The concept just did not exist. Zero samurai of any rank nor any bushi caste warriors in feudal japan used shields. Large armor plates were attached to the shoulders to afford the guy extra protection in place of a personal shield. When ashigaru lower class infantry came along they had no use for portable shields either because they were usually armed with spears.
Fighting on foot was for the low classes of japan their force of ashigaru surrounded the samurai on foot. Samurai typically didn t use shields. The samurai started as mounted archers. Shields in general during that time period for japan were a strange concept actually and if i remember correctly most infantry didn t even use shields.
Manju no wa were covered with kusari chain armour karuta small armour plates or kikko brigandine these armours or a combination of them were sewn to a cloth backing. So basically two handed spears and bows were hugely popular in japan and hand held shields aren t particularly useful for archers and spearmen. When worn the manju no wa looked like a small tight fitting vest. The armour could be exposed or hidden between a layer of cloth.
The samurai s armor adapted accordingly too to this threat. You can see a picture of that here. Since you can t use a bow without two hands personal hand held shields became unviable so they instead latched those onto their armor.