The two acts that turn the plot both belong to Lakshmana, and both are failures of a kind.
The first is the mutilation of Shurpanakha. The demon princess propositions Rama, is passed to Lakshmana, is mocked, attacks Sita, and Lakshmana cuts off her nose and ears. It is a brutal response that later commentators have argued about for centuries, and it is the direct cause of everything that follows, since she goes to her brother Ravana and describes Sita to him.
The second is the moment the abduction becomes possible. Rama goes after the golden deer, a cry is heard in his voice, and Sita orders Lakshmana to go and help. He refuses, knowing it is a trick, until she accuses him of wanting his brother dead so that he can have her, at which he goes, in fury and grief. Later versions of the story add the detail everyone remembers, a line drawn round the dwelling which Sita must not cross and which the disguised Ravana cannot, and which she steps over to give alms. That line is not in the oldest Sanskrit text; it belongs to the later regional retellings, and its popularity is a good example of a tradition supplying the safeguard the story feels as though it needs.
Tradition identifies him with Shesha, the cosmic serpent on whom Vishnu rests, which makes the brother who never sleeps the bed of the god who does.