I found internet sites saying she was Indonesian, one Spanish magazine article which describes her as Burmese, a press release which describes her as Burmese with a British father and Burmese mother and an Italian news clipping which says she was born in London to an English father and Hawaiian mother - so make of that what you will. The press release further says that she was a popular model in Britain and was also a professional dancer.
She first appeared on the screen in 1962 in The Road to Hong Kong, shot in Britain. The following year she shot one film in Spain, then seems to have made Italy her home after going there on a publicity jaunt for Dr. No (in which she did not appear). Seyna is most associated with the eurospy and crime caper genres, in which, being perceived as oriental, she inevitably got cast as villains. When those genres went out of fashion at the close of the 60's, her film work became far more sporadic - she never appeared nude, so probably found work hard to come by once that became the norm.
Since her 2 early uncredited roles are not (currently) documented in any other online filmographies then it could be possible that there are a few more undocumented roles to be found.
- The Road to Hong Kong .... Harem girl (uncredited)
1963 - 55 Days at Peking .... Handmaiden of the Dowager Empress (uncredited)
- Gli imbroglioni .... Exotic beauty (credited)
- Mondo cane 2 .... Cover model for violent pulp novels (uncredited)
1964 - 002 agenti segretissimi .... Agent who seduces Franco (credited as "Selina Seyn")
- I due evasi di Sing Sing .... Nightclub hostess (uncredited)
- I marziani hanno 12 mani .... Girl by the jukebox (credited)
- Sandok, il Maciste della giungla .... Sara (credited)
1965 - Agente 3S3: Passaporto per l'inferno .... Jacky Vein (credited)
- Agente segreto 777 - Operazione Mistero .... Doctor Serens (credited)
- Casanova '70 .... Indonesian airline hostess (credited)
- Giulietta degli spiriti .... Seyna (uncredited)
- Un tango dalla Russia .... Katya (credited)
1966 - Non faccio la guerra, faccio l'amore .... Violetta (uncredited)
- Perry Grant, agente di ferro .... Sonia (credited)
- Ray Master l'inafferrabile .... Princess Ratana (credited)
- Se tutte le donne del mondo... .... Wilma Soong (credited)
- Tabú .... Anahita (credited as "Seyna Sein")
1967 - Flashman .... Flower (credited)
- Le 7 cinesi d'oro .... Vietnamese agent (poster credit - role unconfirmed)
1968 - Niente rose per OSS 117 .... The Major's assistant (credit unknown)
1971 - Riuscirà il nostro eroe a ritrovare il più grande diamante del mondo? .... Lulu Mao (credited)
1973 - Li chiamavano i tre moschettieri... invece erano quattro .... Lee Yang (credited)
- Ming, ragazzi! .... Mig-Not (credited)
1976 - L'unica legge in cui credo .... Zael (uncredited)
1977 - Il mondo dei sensi di Emy Wong .... Brothel Madam (uncredited)
- Pussycat, Pussycat, I Love You (1970) Seyna appeared as a model in advertising for this film, but does not appear in the film itself
- Unknown British TV show with Sammy Davis Jr. - Press release