In Peppa Pig episodes 172-173, "Penny's Come to Playgroup," the following characters appear:

1. Peppa Pig:
   - Gender: Female
   - Voice: Harley Bird
   - Appearance: Main character, appears in all episodes

2. Penny Polar Bear:
   - Gender: Female
   - Voice: Unknown
   - Appearance: Introduced in episode 172, "Penny's Come to Playgroup"

3. Mummy Pig:
   - Gender: Female
   - Voice: Morwenna Banks
   - Appearance: Main character, appears in all episodes

4. Daddy Pig:
   - Gender: Male
   - Voice: Richard Ridings
   - Appearance: Main character, appears in all episodes

5. Doctor Polar Bear:
   - Gender: Male
   - Voice: Unknown
   - Appearance: Introduced in episode 173, "Doctor Polar Bear"

The characters that don't speak in episodes 172-173 are:

1. Peppa Pig in episode 172
2. Mummy Pig in episode 172
3. Daddy Pig in episode 172
4. Penny Polar Bear in episode 172
5. Mummy Pig in episode 171

The characters that are absent in episodes 172-173 are:

1. George Pig
2. Granny Pig
3. Grandpa Pig
4. Suzy Sheep
5. Rebecca Rabbit
6. Danny Dog
7. Candy Cat
8. Pedro Pony
9. Zoe Zebra
10. Emily Elephant
11. Freddy Fox
12. Wendy Wolf
13. Gerald Giraffe
14. Delphine Donkey
15. Mr. Bull
16. Miss Rabbit
17. Mrs. Cow
18. Mr. Potato
19. Mrs. Carrot
20. Madame Gazelle

Please note that the information provided is based on the given episode numbers and may not include any new characters introduced after episode 173.