The antagonists appear to be identifiable by their actions: they hate Christians while claiming to love God (4.20). The writer also suggests they were teachers (2.27), which may mean they had a high profile.
The antagonists are unsaved individuals explicitly denying that Jesus was/is the Christ (2.22-23). Rather, they claim to have fellowship with the Father apart from Christ (1.6). In so doing, they contend that access to the Father and to eternal life may be achieved by works.
Characteristics are not evident.
The readers are undoubtedly saved individuals. They are regularly referred to as children (2.1-13, 18, 28; 3.1-18), they are cleansed from their sins (1.7; 4.10), and they have eternal life (5.11-13).
The writer does not give his name or titles, but clearly identifies himself as a follower of Jesus during His earthly ministry (1.1-5).
The writer is a fellow-believer with the readers and identifies himself as an Apostle (although he does not call himself by that title). He claims intimate involvement in the life of Jesus which is characteristic only of an Apostle (1.1-4). Furthermore, he claims distinctive authority held by the Apostolic office alone (4.6).