Stallbreaker

On surface level, stallbreakers fulfill a similar role to wallbreakers. Both try to hinder walls at doing their job, but the methods are very different. While wallbreakers do it through sheer damage output, stallbreakers stop walls from using their status moves like Toxic, Protect or Recover or set up entry hazards like Stealth Rocks. Some stallbreakers achieve this by virtue of their ability, like for example magic bounce, which simply reflects the status move back at the opponent. Others use moves like Taunt that prevent the usage of status moves in the first place for a set amount of turns, forcing walls to either use their oftentimes single, weak attacking move and preventing them from healing up, or to switch out.

A special subset of fast Pokemon who carry the item Choice Scarf (rarely Choice Specs or Choice Band are used instead) and a move like switcheroo, that swaps items with the opponent. This way, they force the wall to lock itself into a single move until it switches out. These Pokemon usually fulfill this role in addition to their main role, keeping that strategy as one option while primarily using the Choice Scarf themselves to outspeed many Pokemon.

Pivot

Pivots try to force a switch while taking not too much damage, gaining information about the opponent's Pokemon and thus allowing you to pick the right counter to your opponent's Pokemon

Usually a pivot carries some kind of switch-move, like U-turn or Parting Shot. That allows them to immediately switch out again, but just after your opponent switched out first, giving you the advantage in choosing the best match-up.

An important concept around pivots especially is “momentum”: if you manage to force your opponent to react to your turns, keeping them on the back foot, that's a huge advantage as you are then able to dictate the pace of the game. Pivots achieve that by forcing a switch, then directly switching out themselves. This way, your opponent needs to choose their next Pokemon first, giving you the chance to pick the best match-up in return.

Offensive pivots are Pokemon that threaten huge damage or even a KO against your opponent's active one. On the other hand, a defensive one has to be able to shrug off the enemy active pokemon's attacks, forcing it out by simply stopping it from getting anything meaningful done. An especially good ability for both offensive and defensive pivots alike is Regenerator, which heals the pokemon for 30% of its maximum HP every time it switches out.

Hazard Setter und Hazard Control

As mentioned previously, “entry hazards” refers to moves like for example Stealth Rocks. These moves make it so that opponent Pokemon take damage or face another inconvenience (like having their speed lowered or getting poisoned) whenever they enter the battlefield. Hazards are important to stop your opponent from switching out constantly by punishing every switch.

A hazard setter is a Pokemon that knows one of those moves. While most hazard setters tend to be walls, there are some offensive ones as well. On the other hand, “hazard control” refers to being able to prevent or get rid of hazards. The most common ways are Rapid Spin and Defog to get rid of active hazards, while preventing hazards can be done either by preventing the usage of a move like Stealth Rocks in the first place (by Taunting the enemy hazard setter) or with the ability Magic Bounce, that bounces them back to the opponent if a hazard setting move against the Pokemon with this ability.

Revenge Killer

A revenge killer primarily is used to KO an enemy sweeper right after one of your Pokemon was killed. They need high attack or special attack, combined with either being naturally fast, carrying a Choice Scarf or using priority moves like Bullet Punch, which strike first regardless of speed stats of both Pokemon. Many revenge killers can also be sweepers and vice versa, often their role is decided based on match-ups and can also change throughout the match.

Supporter

Supporters do exactly what the name suggests, supporting their team in various ways.

• “Clerics” focus on healing their team, either through moves like Wish for HP healing, or Aromatherapy to get rid of status conditions.
• “Screeners” set screens, which means they use moves like Reflect, Light Screen or Aurora Veil to lower the amount of damage your Pokemon take for a set amount of turns.
• “Annoyers” primarily try to spread status, like burning enemy attackers to lower their damage output, paralyzing them to lower their speed, or poisoning walls to slowly chip away at their health and make the job easier for your wallbreakers.

Oftentimes supporters combine multiple of those roles and/or double as a defensive Pokemon as well.

Roles in general

Last but not least, it's very rare for a Pokemon to only take on one single role. Most fulfill multiple roles, switching between them based on game state and match-up. After all, most roles have at least some overlap with others. Also, it is not necessary to have one Pokemon for every single role on your team, neither is this an extensive list of all possible (sub)roles.