"A Splat Charger modified by Sheldon using some seaweed that was stuck to the blueprints. Maybe the Sprinkler sub-weapon is meant to keep the seaweed fresh?"
Charger-type weapons generally are not used to cover as much turf as the Shooter or Roller weapon types. Instead, they provide cover and/or pressure to high traffic areas of the map, as well as key choke points, allowing the rest of the team to move more freely.
The Kelp Splat Charger will be similar to the standard Splat Charger - you'll want to find a key vantage point to pick people off at, such as the central pillar that overlooks the stage on Blackbelly Skatepark. While you lack a sub that can give a tactical advantage in a skirmish, the Sprinkler helps your team cover more ground, when used right, making it valuable on teams without reliable coverage weapons. The Killer Wail can be used to deal with players just shy of your range or hiding around a corner. It can also help delay the enemy team from advancing - right after you take out an enemy player, wait about two seconds and shoot the Killer Wail at the enemy spawn.
Tips for facing each weapon type
Shooter-type weapons give you more trouble than you would have with another Charger set because you lack a tactical sub-weapon. Instead, be extra careful to abuse your range and not let these players close. If you push them behind a wall or into a tight space they might get the advantage over you, just back off, or find a different vantage point if they're threatening you.
Roller-type weapons are the easiest targets to pick off for Chargers, aside from the Inkbrush. Taking out these players will be your priority, and you can throw a Sprinkler over the area they covered to let you get back to picking off other players, instead of covering where they went.
Charger-type weapons will be all about getting a sneaky shot in. Because you both can hit each other at relatively the same range, you'll be looking for chances to shoot them while they're not looking. Keeping your vantage point is important in this matchup, so your team can stay ahead, but be wary of where you might be shot from if you're not paying attention. Another option is to ignore the enemy Charger and deal with the rest of the enemy team. If you pick them all off, the Charger will have a difficult time holding his ground against four players.
Helpful abilities for this weapon
Cold-Blooded is practically a necessity for this weapon, considering you want to remain as hidden as possible. Ink Resistance Up is great in case you've suddenly lost turf beneath your feet and need to make a faster getaway. Ink Recovery Up or Ink Saver (Sub) are really helpful here as well since you'll want to be using your sub to cover turf primarily.