The first time L saw Light, he didn't think much about him at all. It was in the headshot attached to his file amongst the multitude of other family members of the investigation team.
The second time, was while he and Chief Yagami were watching the hidden cameras in his house. And it was then that L got his first glimpse of Light's intelligence. The way he put a piece of paper in his door to ensure no-one went into his room and found his secret stash of lingerie magazines. Chief Yagami's reactions to this were extreme, and L sympathised with his son, no wonder he made attempts to hide things. He was just looking at pictures of his preferred gender, which was a perfectly normal thing for a boy his age to do.
The way Light made an insightful comment when he learnt about L's lie about the detectives being sent to Japan to work on the case from all over the world further impressed L. It was then that his interest was sparked for the first time by one of the suspects. That didn't mean that Light was most likely to be Kira, L wouldn't let his own personal opinions sway his judgement. And besides it was unlikely. This was the current lead he had decided to follow, but still the chance of Kira being among those under surveillance was merely five percent.
Chief Yagami had mentioned his son in passing. Apparently Light had helped solve a couple of cases in the past. L had secretly wondered if this was his father exaggerating Light's involvement in order to boast about him. Even smart, capable people did that sort of thing. He learnt that Light was consistently top of his class, and that he had scored first in the nationwide practice papers for college admission. That sounded unlikely, but L verified these facts independently. Light seemed intimidatingly normal and well-adjusted for such a prodigy. Perhaps that in itself was suspicious, but perhaps part of L envied Light's ordinary life. Maybe L unconsciously wanted to find something terrible about Light, just to prove that nobody could be that perfect.
The third time L saw Light, was at the entrance exam for To-Oh University. An invigilator loudly told L to sit properly, but L ignored him for two reasons: in his opinion he was already sitting properly, and he was busy staring at Light. According to his father and reports by the late Agent Penber, he was well-liked and popular. And now L had had the chance to observe him in person he could see why. Light was smart and neat, had good posture and was conventionally attractive.
When the invigilator berated L, a few people looked around. Light was one of them. L's heart missed a beat. Now they were staring at each other. This wasn't part of the plan. The others who had turned around went back to their exam papers, but Light carried on staring at him and time stood still. Did Light suspect something? Or was he merely curious about the stranger whose eyes he met by accident?
When they met again, Light was polite and courteous. He accepted L's challenge to a tennis match. It was a close game, and L was impressed that Light was able to beat him. He further impressed him by inviting him to a cafe. No, not just impressed. Light wanted to talk to him further about the Kira case. And that was what L wanted to do too. But in the background, L felt a satisfaction at Light accepting him. It was something one would do with a friend, or at least a prospective friend. Because of the path he had been set on in life, L had had few opportunities to develop friendships. Looking back, he wondered if he wanted to work with Light as an excuse to become his friend. But that wasn't the only reason. Light was smart, devastatingly so. L was pleased to succeed in tricking him with his test, but was still impressed by Light's reasoning. After the tennis match, now they were even.
L wondered how long they would have stayed to talk in the cafe if they had not been interrupted by the news of Chief Yagami's heart attack. Seeing Light's genuine concern for his father reduced his suspicions that Light was Kira, and served to endear Light to L even further. Despite him not being the only suspect, L had found himself unduly fixated on Light from the start. Now L was 100% sure he wanted to work with Light, with a secret secondary goal of becoming his friend.
After the news station airing the Kira tapes and the emergence of the second Kira, L made the decision to ask Chief Yagami if he could bring his son on board. Chief Yagami agreed, if Light did. L felt quite confident that he would.
L had worked on many cases in his career, which meant he had worked with many other detectives and investigators. He preferred to work alone when possible, but involving others was unfortunately essential to solving most cases. From L's experience, even seasoned detectives were of limited use. So he had been expecting the same from Light. Although everything about him seemed promising, L still expected to be disappointed.
Light seemed to immediately adjust to L's way of working. While other people's suggestions were largely useless, Light continued to impress L with his insight. Also L felt comfortable in his presence. Like a friend might.
L was pleased when Light continued to return to work with him. His father aired his concerns about Light spending too much time on the investigation and neglecting his school work and not sleeping enough. L was slightly concerned Light might take those comments seriously and stop helping out, but he knew better than to argue with the chief. It seemed like Light's family were caring, if a little overbearing, and L couldn't help but be a little envious of that. Nobody had ever encouraged L to get more sleep, so he didn't.
Despite his quick progress in narrowing down Kira's location, it seemed that this case would not be one that he would solve quickly. But L had Light's company now, so he was content. The case wouldn't go on forever, but Light was interested in joining the police force, so L was pleased with the thought that perhaps they would work together again.
The case was further complicated when a second Kira appeared. There were similarities, yes, but it was unmistakably a distinctly different killer. The second Kira didn't seem to need a name as the real Kira did. This fake Kira also seemed to kill petty criminals, and those who had not been convicted. L had developed a grudging respect for the original, "real" Kira. He focused on violent criminals, and those who used force against the vulnerable. And those who did nothing wrong other than oppose him, of course. That could not be forgotten about. L felt a growing urgency to catch the second Kira. If he had to die to catch the real Kira, then so be it. But in L's short time profiling the second Kira, he would absolutely not allow them to kill him. Light would help him to catch the second Kira. Light was still a suspect, but L was confident he could rule out Light being the second Kira. The profile did not fit Light, Light was just… more than that.
For the first time in his life, L felt that his life was in danger. He could face down one Kira, but with two Kiras working together, he wasn't sure of his chances. He made the decision to tell the task force that if he died in the next few days, that confirmed Light was Kira. To protect himself, he also told Light himself, and suggested that L was not an individual but a group of people. He didn't like to lie to his friend, but in the event he was correct about Light being Kira, it might save his life.
L saw Light with one of the girls that Mogi had reportedly seen him with. Light had gone on dates with girls before, but now it seemed different. L wasn't sure if his own emotions were clouding his judgement. He liked spending time with Light and a part of him wanted Light all to himself. His dream of two detectives working side by side all day and all night was crumbling.
And then another girl showed up saying she was Light's girlfriend, and she was definitely not one of the ones Mogi had written about in his reports. L didn't need to be introduced, he recognised Misa Misa on sight. L had a hunch that Misa was the second Kira. He had to act fast. The second Kira could kill without knowing a person's real name, and Misa had looked straight at him.
Other people recognised Misa and a crowd gathered around her. L spotted his opportunity and pickpocketed her cellphone. Misa moved slightly as he did so and he brushed against her, but he played it off as a joke, saying he'd catch the culprit who touched her butt. Only Light would understand the other meaning of his joke, but he didn't even smile.
Light calling the cellphone that L had pickpocketed only made him look more suspicious. But before L could make a move concerning him, Light handed himself in, saying he might be Kira despite having no memories of being so. It should have seemed more like a victory to L, but he found himself feeling conflicted about it. Catching Kira had to take precedence over everything else, but this time he didn't want to be right. Everything pointed towards Light, but L didn't want to lose him as a fellow investigator and as a companion. He would do anything to prove that Light wasn't Kira, no matter how long was going to take.