Legendary Heroes[]
Naofumi Iwatani[]
Ren initially holds a low opinion on Naofumi, considering him weak due to his Shield, and a horrible person due to the false rape accusation pressed against him by Malty S Melromarc. His opinion hasn't improved by the next time they meet, as he easily believes that Naofumi had stolen a job from him, however, the realization that he caused a disaster that Naofumi fixed that left him shocked and guilty enough that he immediately ran off to investigate the matter himself.
By the time of the Third Wave, Ren's opinion on Naofumi as a human being has improved, as he is the only one who doesn't berate Naofumi for joining in on the boss fight, and even follows his lead when he realizes how to draw out the boss. That said, he still attributes his Wrath Shield to a cheat. When Melty's life was in danger from the Church of the Three Heroes conspiracy, Ren was the first person willing to believe that Naofumi was innocent of trying to "harm" the princess, and had even apologized for doubting him when Malty had shown her real nature.
However, Ren grows both frustrated and jealous that Naofumi, who can only use a shield, is stronger than him, who uses a sword, to the point that when it comes for Naofumi's time to speak during the meeting of the Heroes, he easily joins in on accusing Naofumi of cheating. Following the Spirit Tortoise's rampage, Ren becomes sympathetic to the plight Naofumi faced when he first came to Melromarc, as he now faces the same persecution at the hands of the same people.
That said, his depression leads him to trust Malty over Naofumi - that had proved to be a big mistake. When Ren falls under the influence of the Greed Series and Gluttony Series of his sword, he comes to see Naofumi only as a high-level boss with rare drops and a high EXP reward, and it takes Eclair Seaetto beating some sense into him to make him see differently.
Now purged of the negative aspects of his personality, Ren starts to see Naofumi as an unexpected source of wisdom and comes to live under him in his village. Since then, Ren has witnessed how hard Naofumi works to help his slaves and how happy they all are living under him, gaining a better perspective of the Shield Hero. When Itsuki was under the Pride Series influence and started accusing Naofumi of working his slaves too hard and keeping the profits to himself (which Naofumi himself agreed on), Ren defended Naofumi by pointing out he was like a parent to everyone in his village, as he was the one working the hardest for their sake's and not the other way around.
The current relationship between the two is very good, both consider each other as a good friend. Naofumi also usually gives Ren some advice so that he can solve his personal problems, such as what he should do with his infatuation with Eclair, how to deal with Wyndia, or his mistrust when forging weapons.
Motoyasu Kitamura[]
Ren tends to avoid Motoyasu, his overly friendly personality and Harem seeker tendencies clash with his loner tendencies. The two are also quick to argue with one another when the information from their games don't line up: such as during the third wave when they were both trying to draw out the Soul Eater and later again during the meeting of the four heroes when they disagreed on the reinforcement methods for their weapons.
Currently, the relationship between the two hasn't changed much, except now Ren tries to avoid Motoyasu even more due to how weird he became after becoming obsessed with Filo. Basically, Ren's current opinion of Motoyasu is very similar to Naofumi's, ie he's an idiot but a useful idiot, in fact he has no complaints when Naofumi says Motoyasu is an idiot, in fact he usually supports that statement aloud.
Itsuki Kawasumi[]
Ren and Itsuki appear to get along amicably. During their downtime, they are shown eating and talking together, and whenever Itsuki accuses Naofumi of cheating, Ren is quick to follow him. Ren was also able to persuade Itsuki to investigate into the charges against Naofumi. However, Ren's trust in his game's information eclipses his trust of Itsuki and when their knowledge doesn't line up they quickly resort to accusations of lying and vulgar insults.
When Itsuki was under the influence of his weapon's Pride Series, Ren showed concern for him as he can easily relate the pain and problems Itsuki was going through.
Vassal Weapon Holders[]
Raphtalia[]
Ren did try to help Raphtalia when he believed that she was forced into slavery by Naofumi, though when he saw how close their relationship was, he was not happy about being duped.
Glass[]
Ren feels anger and resentment towards Glass, who openly despises and mocks him as being too weak and unskilled to be a Hero.
Aultcray Melromarc XXXII[]
Ren holds a slightly bad opinion of the King, while he never treated him as badly as Naofumi, he also didn't treat him as well as Motoyasu, and he also knows that Aultcray was bullying Naofumi by not giving him quests or making excuses to hold back his funding.
Companions[]
Welt, Bakta, Tersia, and Farrie[]
Ren treated his former companions like members in a guild, assigning them areas to level up independently, and having them do errands for him. After they were all killed by the Spirit Tortoise, the immense guilt he felt for their deaths was enough to send him spiraling into depression. After being cured of his Curse Series, Ren comes to blame himself for their demise and seeks to save the world to honor their sacrifices.
Eclair Seaetto[]
Eclair and Ren initially have a bad relationship, stemming from Ren's belief that level is all that is important and that being a Hero automatically made him stronger and more skilled than her. When it comes time to snap Ren out of the Cursed Series, she is the one to volunteer to beat some sense into him and ends up not only beating him twice but also saving him from a pair of Reincarnated Individuals.
When Ren finally comes back to his senses, Eclair is assigned as his training partner and instructor in both the Hengen Musou style and proper swordsmanship. Over time, Ren has developed feelings for Eclair. However, Eclair has been "married" to her work and unable to reciprocate Ren's feelings.
Wyndia[]
Ren has a huge amount of guilt about Wyndia, as he is fully aware of the fact that her status as an orphan and a slave stems wholly from him murdering her father Gaelion, and seeks to make amends. Wyndia, for her part, hates Ren for murdering her father, though she isn't so petty to try and seek revenge on him, she tells him that if he wants forgiveness, then he should save the world.
Gaelion[]
Ren feels guilty for murdering the adult Gaelion, though he isn't aware of the details of his resurrection. The current Gaelion hates Ren to the point that he doesn't even want the Hero to know that he is still alive.