Karthikeya is the son of Lord Shiva. He was born with 6 heads so we call him as shanmukh.
He killed the Taraka and released the Deva and devaths from Taraka’s.
He gives strength, courage and the protective quality of Kuja.
 

So, Kuja is known as the protector of dharma.
Kuja is that ‘Ku’ means for ‘Earth’ and ‘Jha’ means ‘birth’. So, Kuja means ‘The Son of Earth’. 
He is brought up by six Krutika(stars).

Kuja is considered to be a main planet among all planets. 
It gives courage, aggression, vitality, confidence and warrior qualities.
Kuja problems is mainly considered at the time of marriage. 
People will consider Kuja Dosha is a bad yoga.
 

The troubles in marriage are depends on the nature of Kuja. 
Kuja dosha impacts on the 7th house and 7th lord.

It also influencing the Kuja Dosha is the age of the person at the time of marriage. 
For a Kuja dosha people, the malefic effects of Mars reduce after the age of 28 years.
 

Follw Some remedies to reduce kuja dosha 

Frequently Visit Lord Subramanya Swamy temple.
On Tuesday Donate red grams duly tied in a red cloth to the temple priest.
You have to recite Subramanya Stotra for seven Tuesdays, 
Read Subramanya Ashtakam for seven times daily in the morning between 6am to 7am. 
Then you have to recite Kuja Dosha Nivarana Sloka for seventy times.
 
If the Kuja is much serious in your chart like ashtama Kuja, 
Visit Vaideeswaran Koyal temple in Tamil Nadu.
Perform abishekam on the day that Krutika star falls or Shashti. 
Feed food items with red color along with milk and bread to dogs.
In Subramanya Swamy temple, offer red fruits and flowers.
And red dress material to the temple priest.

And wear a pure coral studded ring.
Light a diya at the Kuja idol in Navagraha temple and the wicks should be inred color
Donate 11/2 kilo rice and food items with Dakshin to the temple priest for seven Tuesdays.
Feed the cows red dal mixed with jaggery.
For married women give red colored flowers with red blouse piece. 
Red kumkum to married women on seven Tuesdays.
 
kuja dosh is always problem in marriage life and relations.