Latias. (original Japanese name: Ratiasu. Origin of name and Pokémon unknown.)

Latias and her male counterpart Latios were transformed from a
strange brother and sister pair who had the ability to protect the
village that eventually became the city of Altomere. In the battle
against strange machines of destruction, Latios was killed but his
soul was preserved in a jewel called the Soul Dew. Since the time of
this legend, a pair of Latios/Latias always guard the city of
Altomere, and the Soul Dew is kept at the source of the waters in a
secret garden within the city.

In the 5th movie, Latias is able to move about the city of Altomere
undetected, thanks to her ability to transform her appearance into
one of a human girl. This isn't a perfect transformation, however, as
the thieves Annie and Oakley are able to detect subtle differences in
Latias's body temperature, as well as see through the transformation
with special devices, while attempting to capture this Pokémon.
Latias also has a special affinity for Ash, and leads him to the
secret garden where the Soul Dew is kept. Later, when Latios is
captured to power the "Defense Machine", Latias finds Ash and his
friends to ask for their help. When the machine runs amok and
unleashes a tidal wave on Altomere, Latias flies with her brother to
stop the wave, but it is Latios who sacrifices himself to stop the
oncoming water. Through Latios and Latias's 'sight sharing' ability,
Latias and Ash, etc are able to see what Latios sees as it departs
this world.

Latias can be obtained in:
--Pokémon Sapphire (Lv. 40) only after defeating the Elite Four, in
random and moving locations. If it is knocked out, it will not
reappear, so it is recommended that a Master Ball be used.
--Pokémon Ruby by using an Eon Ticket with the GB e-Reader while
having Mystery Events enabled. Directions for this are enclosed in
the folders in which Eon Tickets are found. In this case, Latias does
not 'wander', so saving before attempting to capture with an Ultra
Ball is recommended for repeated attempts without knockout.