When Koji was a genin, he struggled quite a bit with Chakra Control. Ninjutsu was difficult to pull off, and even when he was successful, it used up more chakra than necessary. Fortunately he found that he was quite talented with throwing shuriken and kunai, and focused on that to find his niche. Kunai ended up being far more versatile and deadly than shuriken, so while the latter weren't abandoned, Koji practiced with his chosen throwing instruments almost exclusively for a long time. He only switched from this path of the kunai when he found that it simply wasn't enough during his first chunin exam.
During the team eliminations, his partners were defeated quickly, and on his own he found just how much he relied on them for his own skills to succeed. Their jutsu, and their teamwork. He fell soon after, and he realized that he needed more than just the ability to throw his kunai. More skills to back it up. It took a lot of effort and training, but slowly he overcame his difficulties with chakra control and ninjutsu. Much of the lessons coming from his family's affinity towards sealing techniques. The ninjutsu he learned even helped develop his techniques with his favored kunai, smooth earth mounds assisting in bouncing them and shadow clones turning to cloning the weapons themselves. By the next year, Koji was ready for exams again.
The exams never happened.
Instead, there was war. Koji was drafted along with all his fellows into the great shinobi war, where he fought and killed again and again. Day after day ended with him covered in blood, either his own, an enemy's, or his comrades. The longer it went on, the more disheartened Koji became with the fighting. It seemed meaningless after a while. Not so much what they were fighting for, but the fights themselves. The breaking point was when he was part of a large operation to take what was said to be a key area. Multiple feudal castles that would serve as bases of operation to move forward on the offensive. The operation was successful, but hard fought with most of the platoon not making it back. And yet, it amounted to nothing. Shortly after the mission, plans suddenly changed. The offense strategy was abandoned in favor of defense due to the shifting tides of war, and the taken castles left to be reclaimed by the land of lightning. The battle had been pointless, and Koji's fellow soldiers died for nothing at all.
Koji was promoted to Chunnin after the mission due to the need of squad leaders, the previous having been killed. With some authority he was able to act to prevent the deaths of those under his lead. He began to take more stock of the missions given, using more care to perform them with minimal losses. A few he outright abandoned for the sake of his soldiers. His actions had very mixed reactions from those higher than him in command. Many approved of his priority of keeping the soldiers alive for future missions, preserving resources, and accurately judging the importance of objectives and how quickly they needed to be completed. On the other side, some thought he was cowardly at best, wasting time and the talents of soldiers, and at worst insubordinate and deserving of court martial.
Koji always walked along the line of orders if it meant keeping his men alive. Sometimes losses were unavoidable, some orders too important, but he always did everything in his power to bring his men home. By the end of the war he'd proven himself worthy of the title of Jounin, where he commanded with similar strategy given to the chunnin under him. He ended his career with the lowest number of deaths of any other commander, while managing to keep up with similar rates of success. Even if a bit shy of the high marks.
Years after the war, he has decided to help train at least a few of the next generation of shinobi. To instill in them the priority to survive, something he feels the academy teaches poorly. So many would say that the mission comes first, without knowing the horrific reality. That is what they must learn.