Lucia Rea Regulus, nicknamed Lu, is one of Touya Mochizuki's wives. Lu is also the third princess of the Regulus Empire and she is currently living with Touya and everyone else in the Duchy of Brunhild.

The second season of Isekai wa Smartphone to Tomo ni. 
 
Touya Mochizuki grows accustomed to his new life in another world. Armed with his trusty smartphone, the teenager accepts small quests at his leisure while spending time with his new fiancées: Yumina Urnea Belfast, Yae Kokonoe, and twin sisters Linse and Elze Shileska. But even in a relaxing environment, Touya is only beginning to understand the responsibilities that come with these engagements.
 
In addition to his romantic woes, the elder fairy Lean wishes to locate the remaining pieces of Babylon, the floating island that the mysterious Professor Regina Babylon created five thousand years ago. Touya reluctantly accepts her request and seeks out the teleportation circles that will lead to the islands. However, strange monsters have emerged—possibly powerful enough to destroy the world. In order to save the second life he was given, Touya must weaponize ancient technologies to fight these monsters—all while balancing his overwhelming number of relationships.
 
(Source: MAL Rewrite)

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After a freak accident involving some lightning winds up zapping him dead, 15-year-old Mochizuki Touya wakes up to find himself face-to-face with God. “I am afraid to say that I have made a bit of a blunder...” laments the old coot. But all is not lost! God says that he can reincarnate Touya into a world of fantasy, and as a bonus, he gets to bring his smartphone along with! So begins Touya's adventure in a new, anachronistic pseudo-medieval world. Friends! Laughs! Tears! Inexplicable Deus ex Machina! He sets off on a journey full of wonder as he absentmindedly travels from place to place, following whatever goal catches his fancy. The curtains lift on an epic tale of swords, sorcery, and smartphone apps!
 
(Source: J-Novel Club)