<Li> Brittany Byrnes as Charlotte (series 2), the new girl who becomes the main antagonist of series two . She enjoys the arts and sciences, but can be controlling and manipulative . She comes into conflict with the girls, especially Cleo, when she begins to date Lewis subsequent to his break - up with Cleo . Upon transforming into a mermaid, she obtains all of the girls' collective powers - clarified as being due to the fact that she was exposed to the transformation conditions on her own rather than the original three being present at the same time . She permanently loses her abilities after a confrontation with the trio . She does not return in series three . </Li> <Li> Craig Horner as Ash (series 2), a keen equestrian and riding coach who appears during series two . He forms a relationship with Emma, but does not learn of her being a mermaid until the season finale, when Emma reveals her identity . Horner does not appear in series three due to filming Legend of the Seeker . </Li> <Li> Taryn Marler as Sophie (series 3), an ambitious and driven character who becomes the main antagonist of series three . She is Will's older sister and takes an immediate dislike to Bella because Sophie sees her as a threat to Will's success as a free diver . She becomes a manager in Rikki's cafe and does as much as possible to cause grief for the mermaids, including destroying Zane and Rikki's relationship . She also tries to get her brother Will to go against Bella by saying she is a distraction . </Li> <Table> <Tr> <Th_colspan="2"> Series </Th> <Th> Episodes </Th> <Th_colspan="2"> Originally aired </Th> </Tr> <Tr> <Th> Series premiere </Th> <Th> Series finale </Th> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> 26 </Td> <Td> 7 July 2006 (2006 - 07 - 07) </Td> <Td> 29 December 2006 (2006 - 12 - 29) </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> 26 </Td> <Td> 28 September 2007 (2007 - 09 - 28) </Td> <Td> 21 March 2008 (2008 - 03 - 21) </Td> </Tr> <Tr> <Td> </Td> <Td> </Td> <Td> 26 </Td> <Td> 26 October 2009 (2009 - 10 - 26) (UK) </Td> <Td> 16 April 2010 (2010 - 04 - 16) (UK) </Td> </Tr> </Table>

H2o just add water season 4 when is it coming out