Jarvan IV is something between melee dps killer like Xin zhao and more defensive Garen. Jarvan also have his Demacian Standard which makes him nice support on the early lane phase and in late game team fights.

There are some other champions that are cool companions to you. Im thinking about auto attackers. Your cooperation will be nice with champions like:
- Ashe
- Tryndamere
- Xin Zhao
- Sivir
- Irelia
- Trundle
- Ezreal - AD/AS build
- Olaf - thanks to Xanker's idea (olaf's armor penetration, his slowing axe and passive will make cool connection with Jarvan)
-and much more

And im not trying to tell you that going with mage to the lane will suck. No it wont but to get as much as you can in your lane phase (AS and Armor buff) you should totally try to pick melee dps.

Pros

- % Armor Penetration in your Dragon Strike, enemy Rammus will hate you.
- Additional attack speed and armor as a pasive for you and spammable same buff for every friendly champion around (stackable for you too) with Demacian Standard.
- Cool escape chase mechanism with Demacian Standard and Dragon Strike right after it.
- Golden Aegis that gives you lot of survivability and slow to enemy champions. Funny thing is that you'll get more hp in you shield if there will be more enemy champions around.
- Cool CC mechanism with Cataclysm (especially if you have someone like Malzahar, Morgana, Cassiopeia, Heimerdinger
- Passive Martial Cadence that works kinda like Madred's Bloodrazor. It hits your opponent for additional 10% of his max HP magic damage every 6 seconds. Cool at creep farm it also does nice dmg in teamfights.
- Great for pushing lanes

Cons

- Champions with good escape mechanism like Katarina with Shunpo or Tryndamere with Spinning Slash will get out of your Cataclysm arene very easy. Walls are thick so almost everyone can get out of there. Same thing with Flash
- BE AWARE of using your Cataclysm in teamfights. I had thing that Annie in my team was running out of gank but she went closer as she saw that we can win this teamfight, she came with like 20% of hp in hope of getting kill while i got on Twitch with Cataclysm she get accidentally inside that arena being focused by enemy champions. That was not clever but you need to be aware of that you can trap your (or even yourself) mate in your 'Arena of Death'
- Jarvan sucks mana like a hell especially in early game. So as soon you will get Wriggle's Lantern make sure to visit Golem everytime he spawns.
- Dragon Strike have IMO bit too short range. If you will throw your Demacian Standard at maximum range you will need to step a bit closer to drag to it with your Dragon Strike
- He sucks at solo lane especially in early game so don't rely much on champion spotlight where Jarvan was kicking Morgana's *** without any problems at middle lane.
- Without cooldown reduction he wont keep Demacian Standard up all the time. Also with Golden Aegis you wont be able to get health boost in teamfight.

Runes are very simple. As he have armor penetration in Dragon Standard you might choose Armor penetration marks for it too. Same with Quints. Seals and Glyphs are up to you. I have dodge Seals and Crit chance Glyphs. But you might try cooldown reduction ones too.

I am using 21/9/0 masteries. As long as you are not pure tank and you want to kill someone. You might need these Offensive masteries much more than Defensive ones. 3 Points in Cooldown reduction to allow you to keep Demacian Standard up all the time where fight goes. If you dont need dodge that much you might choose mana regeneration in Utility tree instead.

I will split whole game to two parts lane phase [which is time from 1 to 6-10 lvl] when you are spending most of your time in your lane. And Teamfight phase [all after when you are wandering around, ganking and teamfighting.

Lane phase

You might start with Cloth Armor and two Health Potion's. It will give you armor. You can start with Long Sword too if you want to go more offensive. Then you go straight for Madred's Razors. And finish your Wriggle's Lantern. As you get it put the ward in the river bushes to warn you soon enough when gank comes. Then go and get your blue buff. You will need golem till you get Philosopher's Stone or Shurelya's Battlesong. Then you go for Ninja Tabi. If you have hard time on the lane (good farmer that dont give you possibility to get enough gold or hard enemies that harras you every time you get closer to minions) go for Boots then build up Philosopher's Stone it will generate gold for you. It's better than nothing.

Gank Phase

When you will have lifesteal, speed and cooldown reduction you might start ganking and so. Last three items depend much on what kind of enemies you have. Generally you will need Force of Nature instead of Warmog's Armor if you will have heavy caster team. You might also try to be a Tank if you dont have one in your team then. Thornmail and Sunfire Aegis might be better instead of Atma's Impaler and Frozen Mallet. You might also think about Randuin's Omen

Jarvan's fight style is based on fast pressing bind keys in teamfights. But... it's much easier than playing for example as LeBlanc. All you have to do is to throw Demacian Standard at your opponents or to the place where they might go in next second. Then pull there using Dragon Strike knocking up oponents by the way. Finally getting additional shield and slow nearby enemies by using Golden Aegis. Now you need to focus on basic attack and keeping Demacian Standard up all the time and moving it to the place where teamfight might move in few moments. Try to stay in the middle of fight you have lots of hp and lots of armor and your Golden Aegis Gives you more hp when enemy champions are nearby.

When attacking turrets. Throw Demacian Standard little bit behind turret at the safe place. It will allow you to have your Demacian Standard pasive doubled and give you ability to fast get out of turret in case of gank from behind. In gank phase dont be afraid of turrets. Throw your Demacian Standard near turret buff up your allies and tank the turrets. You can stay alive without being supported by creeps. Jarvan is also very good for backdooring. Get your fast attacking team mate ( Twitch or Teemo would be great for that) and go for the turret. Throw standard and take all damage while your mate will finnish the turret.

Personally i am using these two summoner spells:

- You all should know why that one. Cool for last hitting enemy champions. Countering healing ones like Dr. Mundo, Tryndamere, Vladimir and dealing true damage.

- To chase and run away from ganks. Its cool when you get to someone with ghost then slow him with aegis

You might also try:

Clarity - As long as he sucks mana like a freak this skill might be usefull at very active lane you will proppably also make your lane mate happy with that skill

Flash - Double chase and escape...

Exhaust - Another slow and cool dmg decrease to overfeeded enemy champs

So... as you proppably see jarvan is not the one who will kill whole team alone. But with some friends he might be really helpfull. And i suggest to be in premade at least at lane. You will never get enough effectiveness while playing solo with a random lane mate.