What we see of Kiritsugu is through memories of Shiro. Shiro's earliest memory of him is during his childhood after being in the explosion during the last grail war. Nearly at death, Shiro sees Kiritsugu's smiling face, then later wakes in the hospital. Kiritsugu decides to adopt Shiro. He teaches him the basics of magic, but intentionally not enough to effectively become a magus. Shiro has little aptitude for magic, so he never moved past the basics either. Shiro recalls Kiritsugu as kind hearted, somewhat flaky and absent minded. Kiritsugu spoke about his dream of one day becoming a hero. On the night before his death, he admits the older he got, the farther he got from his ideals. Shiro declares that he will carry on Kiritsugu's dream and become a hero. It wasn't until the current grail war that Shiro discovered Kiritsugu was not only a magus, but a magus who took part in the last grail war.
However Kiritsugu was a completely different man before Shiro met him, and not the smiling figure he remembers at all. Kiritsugu certainly held the ideal he should be a hero and protect people, however his methods were heartless. Losses are inevitable in war, so it becomes a matter of choosing between how many people you will save, and how many you will sacrifice. To that end Kiritsugu was ruthless. His efficiency in killing to save others seemed like the actions of a machine.
Kiritsugu was known to associate with foreigners and travel to Europe, which is why Shiro's excuse that Saber is a distant relative is actually plausible. Kiritsugu was hired by the Einzbern family to partake in the grail war on their behalf. While the Einzbern's have obsessed over the grail for over a thousand years, they've also come to terms with not being able to get the grail on their own. Thus Kiritsugu was chosen and accepted to the Einzbern family. Only at the end of the last grail war with the grail within his grasp, does Kiritsugu betray them.