If there's anything I disagree with in the advice above, it's the idea that you need the cheat NPC to find Kana in time to prevent L from making a move on her. You can actually find her before she even messages the MC if you pay attention, and the game makes it very easy to do.
First of all, if you go to dinner with her every time, which you should anyway if your goal is to prevent L from getting his hands on her, you basically always start the night in the center location.
From there, the first two things you want to do is 1, go near the NPC that wanders around the area and see what he has to say, and 2 make sure Kana isn't in that one spot around the upper right side of the screen of the screen where L sometimes corners her. The NPC should say something like "Kana? I haven't seen her." if she's not in the area. Though he might say "I think I saw her go to the West side." or any number of things which will generally tell you where to go. If the NPC has seen her, just follow the clue he says and go to that part of town to find her, and pass by the NPC in that area to see what they say while inspecting around.
If the center NPC has NOT seen Kana, go to the right side of the town first, aka the East Side, and pass by the NPC there to see if he's seen Kana, and of course check the spots around without entering any buildings. If this NPC also hasn't seen Kana, what you do next is exit the map on the right side, then loop around to the left, aka the West side of the town, where she'll likely be.
Why do it like this? Because, due to a quirk of the time keeping system of the game, you spend less time exiting the town and re-entering on the West Side, than by passing through the Center and then to the West Side. Dunno why the game passes less time like that, but doing it like this basically means that you'll often find Kana before she even sends the message, regardless of what she's doing at the time.
If anything, once you understand how the game works, it's TOO easy to cock-block L.