Kumbakarna

A might gaint.
Kumbakarna was powerful and strong. He was invincible. He was not evil, but his blind love for his brother could make him defeat anybody in battle single-handed.

Coming to the question: What does Kumbakarna signify in Ramayana or in life?

We need to go back to the story to answer this

Kumbakarna performs a severe penance to ask for a boon of complete invincibility. Fearing Kumbakarna's power, valor and invincibility, the Devas plead a Goddess to help them break Kumbakarna's penance. She manipulates him (his mind) and makes him ask for a boon of being able to sleep 6 months continuously!!

Now coming back to the significance. Kumbakarna's story, like many other stories in Hindu mythology seeks to tell us the importance of almighty. It tells us no matter how invincible someone might feel she or he is, one is powerless in front of forces of nature / calamities / The Energy above us / God (God since it is a mythological story).

Depsite being a demon, Kumbhakarna was considered so pious, intelligent and brave that the king of demi-gods, Indra, was jealous of him.

 Along with his brothers, Ravana and Vibhishana, he performed a major yajna and penance to please Lord Brahma. When the time came for asking a boon (blessing) from Brahma, his tongue was tied by goddess Saraswati (acting on Indra's request). So instead of asking for "Indraasana" (seat of Indra), he asked for "Nidraasana" (bed for sleeping).

It is also said he intended to ask for Nirdevatvam (annihilation of the Devas) and instead asked for Nidravatvam (sleep). His request was granted by Lord Brahma. On realizing the goof-up, Ravana asked Brahma to undo this boon as it was in reality a curse. Kumbhakarna slept and was awake for six months each. On waking up after 6 months he use to be so hungry that he use to eat everything in the vicinity, including humans.

During the war, Ravana went into battle and was humiliated by Rama and his army. He decided that he needed the help of his brother Kumbhakarna, who was then awakened with great difficulty. He woke up only after one thousand elephants walked over him.

When he was informed of the circumstances of Ravana's war with Rama, he tried to convince Ravana that what he was doing was wrong. However, he chose to fight in the battle due to his loyalty to his brother.

 After becoming drunk, Kumbhakarna went into the battle field. He devastated Rama's army, injured Hanuman and knocked Sugriva unconscious and took him as a prisoner but was killed by Rama. When Ravana heard of his brother's death, he fainted and proclaimed that he is truly doomed

Able to realize the mistakes and wrongdoings of Ravana, even trying to intervene and moderate at certain times, he is unable to openly oppose him, feeling bound by his warrior ethics. Fighting on what he knows is the wrong side, he is ultimately killed in combat. When he was finally defeated, he annihilated in Lord Rama and got salvation.

Kumbhakarna's character can be compared to Vikarna, the brother of Duryodhana from the Mahabharata, the other great Hindu epic. Both knew that their brothers were wrong, but still fought for them as a mark of duty. The battle between Kumbhakarna and Rama is prominently featured in the Balinese Kecak dance.