To "walk by" someone means to walk past them. These would all count as "walking by" someone:

• Person A is standing in a hallway, and Person B walks down the hallway past Person A's position.
• Person A is sitting on a bench facing a sidewalk. Person B walks down the sidewalk toward the bench, then continues walking past the bench.
• Person A and Person B walk toward each other, then continue on past each other.

You can use "walk by in the following ways:

Yuichiro just walked by.

Brendan walked by me without saying 'hello'.