Sebastian, Viola's twin-brother, was saved at the wreck by a caring & courageous sailor, Antonio. We see them together in conversation on the coast of Illyria in act2 sc.1.The scene shows how Antonio has conceived a strong liking for Sebastian. Although Illyria is not at all safe for Antonio, he follows Sebastian.
In Act3 sc.3, we again see Antonio with Sebastian on a street in Illyria. Antonio knows that the place is likely to endanger him, but his passion for Sebastian, 'more sharp than filed steel', compels him to be with his friend who happens to be a stranger to Illyria. Antonio lends Sebastian his purse, and goes off in search of a lodging, while Sebastian decides to feast his eyes with 'the memorials and the things of fame/That do renown this city'. Sebastian is expected to meet Antonio at the lodge called Elephant in the south suburbs of Illyria.
In act3 sc.4, when Sir Andrew & Viola, as Cesario, are about to draw their swords to fight a duel, Antonio enters. He mistakes Cesario for Sebastian, and wishes to fight Sir Andrew in his place. However, the officers appear on the scene and Antonio is arrested in connection with the charges filed against him by Duke Orsino.
Antonio has been presented as a courageous & and adventurous sailor who had once fought a sea-battle against the Illyrian Duke, for which he is wanted in Illyria. but he was so strongly attached to Sebastian for some months that he was unwilling to part with his friend. He followed Sebastian into the town of Illyria, arranged for his friend's lodging, gave Sebastian his purse in case he needed some money,and was much concerned for Sebastian's safety in an unfamiliar location.