The Reincarnated Prince and the Twilight Knight (Volume 2) by Nobiru Kusunoki

Prince Herscherik (previously the 35-year-old Japanese woman Ryoko Hayakawa) continues to fight against the powerful corruption in his kingdom and the puppet-master minister Barbosse. Now that he’s 5, Herscherik earned his own room and office among the other princes, his new base of operations. On the HR front, he gains a new ally in addition to Kuro, the former-spy butler, in the form of a delinquent but more-than-capable knight from a well-respected family, Octavian. As Kuro and Octavian clash, Herscherik is left to keep them in check as they further their cause to save Gracis Kingdom and its people.