3. Name 10 series where the actor or actress plays more than one character. Also name the actor/actress and what characters they play. (You can describe it too.)

Legendary Four Aces- Esther Kwan- played the assassin older twin and the maiden younger twin
Hap Hak Hung-Tong Leung- played the playful twin and the more serious twin (can't really describe them, never watched this series)
Demi-Gods and Semi-Devisl 90's- Felix Wong- played Sui Fung and Sui Fung's father
Demi-Gods and Semi-Devisl 90's- Carmen Lee- played Lau Yu Yin and her mother and grandmother
A Step into the Past- Raymond Lam- played Poon Yee and the imposter
A Step into the Past- Sonija Kwok- played modern Louis Koo's girlfriend and ancient Kam Ching
Detective of Investigation 3- Kenix Kwok- played Jessie and the girl that looks like her in Beijing
Duke of Mount Deer 2000- Athena Chu- played Ah Or and the mother
Time Before Time- Gordan Lam- played Dai Gong and Lok Wan Gong
A Ching Emperor- Lau Sui Bing- played the princess older twin and the maiden younger twin
(just in case Athena Chu doesn't count) Demi-Gods and Semi-Devisl 80's- Cecilia Wong- played Ah Chu and Chong Ling

question: how specific do you want the "masked" characters? like does Leila Tong counts when she was wearing her cloth at the beginning of the story?

sorry, forgot to say the series name: Twins of Brother

wow, how many times have you helped me, raviolie?  it's great to know that there are people out there that are willing to help

UPDATE ANSWERS:

1. Who was in Secret of the Heart, A Matter of Customs, Mutual Affection, and War of Genders?

- Joyce Chen (?)

2. Name 10 series where there's a masked person. Also, name the actor/actress.

War of In-Laws- Bosco Wong
Return of the Condor Hero 90's- Maggie Cheung (after Wong Yurt Sin took her as a pupil)
Return of the Condor Hero 90's- Louis Koo (after Sui Long Lu left him for 16 years)
Return of the Condor Hero 80's- Andy Lau (after Sui Long Lu left him for 16 years)
Twins of Brother- Derek Kwok (was the "evil emperor's" pupil)
Demi-Gods and Semi-Devils 90's- Felix Wong (as Sui Fong's father)
The Gental Crackdown- Michelle Yim (Nikki Chow's mother)

The Vigilante in the Mask- Deric Wan

Blade Heart- Adam Cheng

Blade Heart- Raymond Lam

Old Time Buddy 2- Maggie Cheung

3. Name 10 series where the actor or actress plays more than one character. Also name the actor/actress and what characters they play. (You can describe it too.)

Legendary Four Aces- Esther Kwan- played the assassin older twin and the maiden younger twin
Hap Hak Hung-Tong Leung- played the playful twin and the more serious twin (can't really describe them, never watched this series)
Demi-Gods and Semi-Devisl 90's- Felix Wong- played Sui Fung and Sui Fung's father
Demi-Gods and Semi-Devisl 90's- Carmen Lee- played Lau Yu Yin and her mother and grandmother
A Step into the Past- Raymond Lam- played Poon Yee and the imposter
A Step into the Past- Sonija Kwok- played modern Louis Koo's girlfriend and ancient Kam Ching
Detective of Investigation 3- Kenix Kwok- played Jessie and the girl that looks like her in Beijing
Duke of Mount Deer 2000- Athena Chu- played Ah Or and the mother
Time Before Time- Gordan Lam- played Dai Gong and Lok Wan Gong
A Ching Emperor- Leung Sui Bing- played the princess older twin and the maiden younger twin
Demi-Gods and Semi-Devisl 80's- Cecilia Wong- played Ah Chu and Chong Ling

Duke of Mount Deer 98- Leung Sui Bing- played Ah Or and the mother

The Hitman Chronicle- Leung Sui Bing- played the princess and the assassin

Reaching Out- Kenix Kwok- played Hui Gwun and Hui Ah

...........sorry, don't know what characters Liza Wong and Angie played.