There　are　two　purposes　in　this　paper.　First,　it　presents　an　approach　to　designing　an　agent　For　adapting　to　the　cooperative　requests　of　the　agents　in　a　multi-agent　system,　we　distinguish　between　the　two　aspects　of　underlying　attributes　of　an　agent:　individual　and　organizational.　And　introduce　a　concept　＇pairs　of　roles＇.　The　other　purpose　is　to　show　how　to　express　the　behavioral　semantics　of　an　agent　and　a　multi-agent　system　with　the　＇state　transition　graph＇.　The　individual　attributes　of　an　agent　consist　of　beliefs　and　intentions,　and　for　intentions　we　adopt　the　＇twin-subset　semantics＇　developed　by　us.　While　the　organizational　attributes　of　an　agent　consist　of　＇pairs　of　roles＇.　These　compose　the　state　of　individual　agent　and　the　relations　between　agents.　The　state　transitions　are　produced　by　the　behaviors　of　agents　and　materialize　the　dynamic　characters　of　agent　and　the　turn　of　a　multi-agent　system.　And　the　behaviors　arise　from　the　mental　states　of　agents　and　the　situations.