A bit of downer
Saruhiko comes off as withdrawn and surly at first glance. He is often seen scowling or grimacing at things that annoy him. It should be noted of course that most things annoy him. This all changes the moment he goes into battle he makes a complete turn and becomes manic, sadistic, and cruel. These mood swings make him hard to get to know and at both Scepter 4 and Homra he begins as an outsider.
It isn't until people spend more time with Saruhiko that they realize he is antisocial but not a lost cause as far as friendship is concerned. He will often berate members of the Special Operations Squad for filing incomplete reports. He will make them refile them when they are beyond repair but he is also willing to complete their reports and answer their questions even when he thinks they're annoying or foolish. He feels that working overtime on the reports of others is better because he can do it more efficiently. He doesn't see the benefit of teamwork and others will often have to intervene when he overworks himself. Despite the fact that he often picks up others work for himself will complain about it.
In the field Saruhiko has a habit of bending the rules. He will go off on his own to obtain information that would not be completely available through official channels in order to save time. He will enter into battle after picking a fight by mocking his opponents. While he normally presents an arrogant and holier than thou front during his battles he is not above mocking himself to get a rise out of others. He comes off as enjoying battle and often seems happiest when he is in the thick of it.
It's difficult to believe that Saruhiko was always so withdrawn however his failure to take care of his body's needs (he subsists off of protein bars and energy drinks) and his insistence on hiding his injuries (See K: Memory of Red Ch. 11, Pg. 17) implies that Saruhiko either has a serious mental disorder or remains hurt by the abuse he suffered as a child.
There is one scenario that I see as being more likely. His father comes off as a sociopath and while there is no direct genetic link there are genes that cause an increased risk. Saruhiko displays some of the same behavioural traits though he only displays the majority of them during battle with Yata Misaki. Therefore, it is more likely that he could have become withdrawn due to the lack of human contact when he was a child. His rage and twisted reactions during battle coming from the abuse he took from his father or in an effort to distance himself from Yata. The lack of parental presence could also attribute for his inability to properly care for himself or get proper nutrition.