Now that we know Locke is the one that sent Richard to visit his younger self, and that Ellie is probably Ms.Hawking who helped Desmond make the right choices to get to the island, all signs are pointing to Abaddon being an other also. Abaddon was instrumental in convincing Locke to go on a walkabout, which led him to the island. Now with the time travel, if they meet Abaddon, Locke will be able to tell him to do this. This also explains how he is connected to Widmore, if they were both 'others', and are now working together, although I believe even if Abaddon is working for Widmore, he is secretly working against him. While Widmore sends the freighter team to find and take the island for him, Abaddon knows that they need to be sent and their presence on the island will probably eventually be vital to saving it.