Tadatsugu Sakai (酒井 忠次, Sakai Tadatsugu) is the president of Musashi Ariadust Academy, and formerly one of the Four Heavenly Kings of the Matsudaira. Though somewhat aloof, he still maintains his power.
Tadatsugu's image is similar to a man who is past his prime: his hair is grayish, his eyes are droopy, and he slouches his back as he walks along the Musashi. However, looks can be deceiving as he is still has the strength to carry two adult-sized automatons and he can still outwit a well-trained fighter.
His outfit consists of a mint-green robe covered by a light blue haori and comfortable sandals. He is also seen smoking through a kiseru.
Despite being a president, Tadatsugu appears to be laid-back, spending most of the time lounging around the Musashi and watching over the different things going on inside the flying ship. He sometimes gets scolded by "Musashi" because of this. However, he is also affected when things become serious.
Tadatsugu is also a supportive figure in Musashi Ariadust Academy. He knows of the story behind Toori Aoi and Remorse Way, and he gives him advice if the need arises.
During his days as a student, he served under Motonobu Matsudaira, then Eternal Student Council President of Musashi Ariadust Academy, alongside Tadakatsu Honda, Naomasa Ii and Yasumasa Sakakibara, who also hold key positions in the school's Student Council.
He later had a certain scuffle with Pope-President Innocentius about 20 years ago, when Innocentius tried to make a push for Catholicism on the Musashi and Sakai and the rest of the student council at the time rebuffed this attempt. The losses made as a result of the failed venture had lasting negative effects on K.P.A. Italia, causing Innocentius to hold a grudge against Sakai ever since.
Despite being a man of age, Tadatsugu still has some of his ability he had when he was still a student of Musashi Ariadust Academy. He once utilized his superior speed and reaction time against Futayo Honda, who ends up having her butt grabbed by Tadatsugu during his visit in Mikawa.