Tensei shitara

Veldora Tempest: Best friend and adoptive older brother. They met after Rimuru's reincarnation into a Slime in the cave where Veldora was imprisoned by the True Hero after he burned down a city.
Ciel: Rimuru's companion since the beginning of his reincarnation. At the beginning of the novel Ciel, known as Great Sage and later Wisdom of Lord Raphael, is a being called Manas. Throughout the story before becoming what she is now Ciel did not have an ego, a soul, and therefore had no feelings or emotions for anything. It was thanks to Rimuru and their great relationship between them that Ciel awakened an ego to serve her Lord. After a time Rimuru gives her a name and at that moment Ciel develops a complete ego and a great devotion to her Lord. The ego that Ciel develops is similar to Rimuru's soul since it can be seen that both have a great resemblance. At the end of the novel it can be implied that Ciel has feelings for Rimuru showing the other girls who want to get close to him that she is the only one who can be his companion and partner.
Veldanava: Creator god of the universe and the first True Dragon. Veldanava and Rimuru have never met, but it is implied at the end of the web novel that Rimuru may be Veldanava's reincarnation or his successor.
Izawa Shizue: Rimuru's destined persona. A young Japanese girl who was summoned in the middle of a war in Japan.
Chloe O'Bell: One of Rimuru's students when he goes to the Royal Capital. Chloe sees Rimuru as a love interest which she does not hesitate to show even though she is a child. When Rimuru goes to save her from her overflow of magical energy a spiritual being from the future appears (which turns out to be Chloe herself). Later it is revealed that Chloe has the power to travel in time through her memories and as long as time is flowing. Chloe could see countless times how her beloved Rimuru was killed over and over again in other futures.