Castalius rosimon is fast and acrobatic, usually seen flying alone or in small groups with 2 or 3 butterflies.
   They visit low growing flowers, and their wings are left upright in sunny weather.
   In the afternoon, they are often seen perched on the grass, and when the weather is cloudy, it will be sunbathing with their wings half open.
   They are quite fast but irregular flyers, and fly around shrubs and herbs.

   They are fond of feeding on nectar of those plants and do not feed on bird droppings like the Banded Blue Pierrot.

   They are commonly found in open vegetation up to the lower hills including wastelands, roadside vegetation and scrublands. In forests, it only appears in sunny openings.

   Both sexes are identical except for the smaller area of blue at the wing bases on the dorsal side on females.

   The pattern on both sides can vary. The cilia of both wings are alternately marked in black and white. Their outer margins are marked in black on the dorsal side with a row of white lines on the hindwing. The inward area is white with intrusions of black. The black patches on its ventral side are evenly distributed throughout the wings, without leaving any distinct patchless areas.

   The forewing has a black basal line which continues up to its thorax.

   The patches on the sub marginal bands are not apparent against the other spots.

   The sub marginal spots near the tornus are suffused with brilliant, shiny light blue scales in newly emerged individuals.