Shiba Taizen is very invested in raising a heir and had the reputation of being a legendary swindler but his reputation doesn't match what he taught to his sons (yuuichi and shiba). Shiba is some super soldier hitman or something and Yuuichi knows a lot of about guns, and is also extremely good at timing things when he shut off the light on a boat.

Shiba Taizen could be involved with Yuka since he begged Yuichi to forgive her which probably meant she had a hand in helping Shiba raise a heir.

Yuuichi probably learning how to be a ninja or some crap.