Chell’s Neutral Special is the Portal Gun. By holding the Special Attack button down, she will aim her shot. Releasing the button will cause her to fire the shot creating a portal where it lands. The first portal does nothing until the second portal is created, which creates a hole in the surfaces they appear on. They’re big enough for even the tallest characters to walk through them (I had originally advocated for smaller portals, but now I realize that they kinda have to be large enough for characters like Ridley and Sephiroth to fit through them), and they allow you to fall in one hole, and come out the other.
The portals are color coded based on which player slot Chell is on. Player 1 has blue and orange portals, Player 2 has cyan and dark indigo portals, Player 3 has dark yellow and light green portals, Player 4 has green and lavender portals, Player 5 has yellow and brick red portals, Player 6 has white and dark blue portals, Player 7 has pink and red portals, and Player 8 has purple and black portals. Since players need to know where all portals lead rather than just their own, computer opponents will also adhere to these rules instead of getting grey ones.
The shots fired by the Portal Gun can hit opponents, but it doesn’t do any damage or create a portal there. Portals also can’t be created on grated surfaces (the shot goes through them), non-flat surfaces, moving surfaces, surfaces smaller than the portals, or too close to other portals when both are going onto the floor. Exceptions to these rules are where the main platform has these characteristics, so you can place portals on Lylat Cruise even though the entire stage is constantly moving, Mario Galaxy even though the entire stage is round, and so on. If a portal is placed on something that is later destroyed or begins to move, the portal will vanish. Portals can be placed on the bottom of semi-solid platforms (and the glow from them will extend so as to remain visible), and doing so does not cause players that drop through them to go through them.
Nearly everything that moves is able to go through portals. This includes players, projectiles, items, and certain stage elements (though these are almost exclusive to the Test Chamber S and Stage Builder stages). Any momentum is preserved through the portals, but the trajectory is changed depending on which way the exit portal is facing (which is always the opposite direction of the terrain the portal is on). For example, if Fox shoots a laser into a portal on the wall that has an exit portal on the floor, it will come out of the exit portal going straight up, and the same will happen if Fox runs in.
As stated before, portals act as holes that allow you to go in one side and out the other, which means that you can stand in two places at once. For example, if Bowser was to stand in between portals, he can clearly be seen both where he’s entering and where he’s exiting the portal, and he can be hit in both locations as well.
Depending on how entry and exit portals are situated, a characters and items may come out upside down or sideways. If this happens to a character (this includes Assist Trophies), they will quickly flip right side up, and any attacks they were doing will launch in the direction relative to their orientation as they do so. If a character is doing a downward charging move, the move will immediately end as they flip. This includes stall and fall Down Aerials, Down Specials like Yoshi Bomber, Up Specials like Climhazard (when going down), and other special moves like Backslash. Certain other Special Moves will end as well if the orientation change would mess them up.
As stated before, momentum is preserved upon going through a portal, which means that the faster you go into a portal, the faster you will come out of one. This is important to note, because you do not go faster when you come out of a portal, you can go faster than normal in a given direction depending on how you enter it. For example, if Fox fast falls into a portal on the ground, he will come out the portal on the wall at a rate faster than his normal air speed.
One last thing to note is that Chell will fire the portals in a pattern of the first one, then the second one, and then back to the first. If the player wishes to skip one in this pattern, then they can hold the shield button as they aim the Portal Gun.