Saji and Issei have a friendly rivalry going on because they are both Pawns, they both harbor feelings for their masters, and they both have Dragons in their Sacred Gears. At first they did not care for one another (due to Issei being one of the Perverted Trio and Saji's cocky attitude), but when the "Excalibur Destroying Squad" was formed, the two shared their biggest dreams and eventually found out that they are alike one another, in that they share each others desire to marry their masters. Their relationship to each other is strong enough that Issei even went as far as describing them both as "War Buddies". Their friendship developed further as the series progressed, as Saji expressed sadness and grief when Issei died, even promising to avenge him.
Despite being good friends with one another, Saji is very envious and jealous of Issei, even to the point of seemingly developing an inferiority complex towards him. While Issei has made tremendous growth in strength and has made his master his girlfriend, Saji has felt left behind in comparison. He laments the fact that he will never surpass Issei in power, no matter how hard he tries. It is not until he states his dream and finally obtains his Balance Breaker that he stops comparing himself to Issei.
Like any other boy at their school, Saji disliked Kiba for being a good looking and popular with the girls. After being forced into helping destroy the Excalibur swords for his sake and upon being told about his tragic back story, Saji teared up so much and felt sympathy for Kiba that he let go of previous grudge and began to willingly assist them in their mission.
Saji is a Pawn in Sona's Peerage. Sona is also the woman that Saji is in love with. He has even stated a desire that if he could ever sleep with her, he would try to get her pregnant so that they would be forced to marry. Sona, while having some affection for him, has demonstrated no romantic interest in him, preferring men who are at least her equal in intelligence. She states that she sees him more as a little brother, and that she does not want to touch him when she knows that two other members of her peerage have shown romantic interest in him.
When Saji joined the Student Council, he regularly gave her helping hand with work whenever she needed it. From this point Ruruko had developed a crush on Saji and has a love rivalry with her senior Momo Hanakai, who is also in love with Saji. Though Saji hasn't the slightest awareness of this due to being infatuated with Sona.
Momo has a crush on Saji whom she affectionately calls "Gen-chan". She initially had no interest in Saji, rather she was more into Yuuto Kiba, but unknowingly began having feelings for Saji after witnessing his passion when he fought against Issei and the constant endless training he went through. So she set her sights on competing against Ruruko on winning Saji's affections. Saji also doesn't appear to completely reciprocate her feelings since his love is still loyal to Sona.
Another member of Sona's peerage that he has gotten closer with over time after joining. She has a deep trust in Saji as she gave him words of encouragement when about to face the members of the Khaos Brigade.
A fellow member of the Sona's peerage whom Saji has gotten close to after joining. Tomoe regularly trains Saji especially when it comes to speed and escape skills, thus she warns him to run if things get too dangerous.
Saji appears to be close in general with Reya as members of Sona's peerage. Reya worries for him before heading into battle but has high hopes that he'll come back. She also refers to him as "Gen-chan" but in a teasing manner.
Vritra is one of the Five Great Dragon Kings known as the Prison Dragon who resides inside of Saji's Sacred Gear(s). Their relationship is similar to that of Issei and Ddraig's, they get along just as well as Vritra considers Saji as his "other half".