it wasn't mentioned in the manga but in the novel, House Sopeid despite their really haughty and arrogantly high pride are actually a House way too competent once they drop their attitude... as if their attitude changed with a switch. This arc will shed a light about that. They are pragmatic, straight to the point with no nonsense bullshit in their actions and basically Emiya Kiritsugu on crack, and that earned Sansui's loyalty alongside that they love Rain and is raising her.