I thought it would be interesting to keep track of which athletes have origins from other countries, even if just a small percentage.
I'll start with the relevant people who will likely or possibly be competing in the 2020 Olympics from Greece
Represent Greece but have non-Greek origins or partial non-Greek origins:
Antetokounmpo brothers (basketball) - Born and raised in Greece to Nigerian parents
Nick Calathes (basketball) - Born and raised in USA. His paternal grandfather was Greek.
Tyler Dorsey (basketball) - Born and raised in USA. His maternal grandfather was Greek.
Zach Auguste (basketball) - Born and raised in USA to a Haitian father and a Greek mother
Elina Tzengko (athletics - javelin throw) - Born and raised in Greece to Albanian parents
Eleni Polak (athletics - pole vault) - Born and raised in Greece to a Polish father and a Sri Lankan mother
Emmanouil Karalis (athletics - pole vault) - Born and raised in Greece to a Greek father and a Ugandan mother
Konstantinos Douvalidis (athletics - hurdles) - Born and raised in Greece to an African-American father and a Greek mother. Uses mother's surname.
Alexi Pappas (athletics - long distance/marathon) - Born and raised in USA to a Greek father and an Italian-American mother. Her mother died when she was a baby so she was basically raised by her Greek grandparents.
Gloria Privileggio (athletics - long distance/marathon) - Born and raised in Greece to an Italian father and a Greek mother
Tatiana Gusin (athletics - high jump) - Born in Moldova but moved to Greece at a young age.
Kristian Gkolomeev (swimming) - Born in Bulgaria to Bulgarian parents, but raised in Greece since he was a baby
Ilektra Lebl (swimming) - Born and raised in Greece to a Czech father and a Greek mother
Stefanos Tsitsipas (tennis) - Born and raised in Greece to a Greek father and a Russian mother
Byron Kokkalanis (sailing) - Born and raised in Greece to a Greek father and a Filipino mother
Aspasia Christodoulidis (rowing) - Born in Greece to a Greek father and a Russian mother. She lived in Athens for many years, but her father is a Greek diplomat so she lived in many different countries growing up.
Zoi Fitsiou (rowing) - I'm not sure where she was born (probably Greece) but she was raised in Greece. Her father is Greek and her mother is Syrian.
Palaiopanos brothers (rowing) - Born and raised in Greece to a Greek father and a Greek-German mother (their maternal grandfather was German). Their mother Tonia Svaier was born and raised in Greece to a German father and a Greek mother, and she rowed for Greece at the 1988, 1992 and 1996 Olympics.
Maria Alziguzi (artistic swimming) - Born and raised in Greece to a North African father and a Greek mother. I don't remember which country her father was from, I think Algeria. Or maybe Libya?
Leon Laokratis Kessidis (wrestling) - Born in Sweden. He has a Greek surname but I have no idea if both of his parents are Greek or only one. His brother competes for Sweden (see below).
Do not represent Greece but have Greek origins:
Albania: Evagjelia Veli (weightlifting) - Born in Albania and represented them in Rio 2016. Her name is Greek and Wikipedia says she has both Albanian and Greek nationality. She also seems to live in Greece based on her Instagram and writes in Greek. The city where she was born in Albania is not part of the Greek minority zone, but it is close by. I don't know any more details about her.
Australia: Theodore Benehoutsos (swimming) - Born and raised in Greece to Greek parents. Has been living in Australia since he was 15 (2013). Used to be known as Theodoros Benechoutsos, but I guess he changed his name to sound more Australian. Represented Greece until 2017, now representing Australia.
Australia: Thanasi Kokkinakis (tennis) - Born and raised in Australia to Greek parents
Australia: Nick Kyrgios (tennis) - Born and raised in Australia to a Greek father and a Malaysian mother
Austria: Alexandri sisters (artistic swimming) - Born and raised in Greece to Greek parents. Moved to Austria as teenagers. Used to represent Greece.
Bulgaria: Grigor Dimitrov (tennis) - Born and raised in Bulgaria to a Bulgarian parent and a half Greek half Bulgarian parent. Played against Greece in Davis Cup in Athens in 2014 and said in an interview that he loves Greece because his grandmother was Greek.
Canada: Joseph Polossifakis (fencing) - Born and raised in Canada to Greek parents
Cyprus: Paraskevas Batzavalis (athletics - javelin throw). Born and raised in Greece to Greek parents. Represented Greece internationally until 2018. Missed qualifying for Rio 2016 because he achieved the qualification standard after the qualification deadline but before the Olympics began. Now competing for Cyprus. His wife is a Cypriot high jumper (Leontia Kallenou).
Cyprus: Dimitrios Chondrokoukis (athletics - high jump). Born and raised in Greece to a Greek father and a Greek-Cypriot mother. Won gold for Greece at the 2012 World Indoor Championships, but missed London 2012 because of a doping ban. Came back and represented Cyprus at Rio 2016.
Cyprus: Antonis Martasidis (weightlifting) - Born and raised in Greece to Greek parents. Used to compete for Greece and then switched to Cyprus and represented them in Rio 2016. He got a doping ban so I'm not sure if he can compete in Tokyo or what the suspension was.
Cyprus: Ramona Papaioannou (athletics - sprints) - Born in Greece to a Greek-Brazilian father who played on the Greek national football team. I am not sure who her mother is or where she was raised, but she competed for Cyprus in Rio 2016 and was training in Greece for many years.
Germany: Sideris Tasiadis (canoeing) - Born in Germany to Greek parents but lived in Greece for 10 years during his childhood. Won a silver medal in London 2012 for Germany instead of Greece
Great Britain: Constantine Louloudis (rowing) - Born in Great Britain to a Greek father and a British mother. Won bronze in London 2012 and gold in Rio 2016 (for Great Britain).
Netherlands: Afrodite Zegers (sailing) - Born and raised in Greece to Greek parents. Her name used to be Afrodite Kyranakou before she got married to a Dutch guy. She got 4th place in Rio 2016 for the Netherlands. Used to represent Greece.
Russia: Elena Vesnina (tennis) - Born in Ukraine and raised in Russia to a Russian father and a Greek-Russian mother. Her maternal grandfather was Greek. She won gold for Russia in Rio 2016.
Spain: Artemi Gavezou Castro (rhythmic gymnastics) - Born and raised in Greece to a Greek father and a Spanish mother. Competed with the Greek national team until 2013, when she moved to Spain for her studies. Won silver with Spain at Rio 2016. Not sure if she is still active.
Sweden: Alex Kessidis (wrestling) - Born in Sweden. He has a Greek surname but I have no idea if both of his parents are Greek or only one. His brother competes for Greece (see above).
Sweden: Jiannis Smalios (athletics - javelin throw) - Born in Sweden to Greek parents. I am not sure where he was raised, but he represented Greece internationally for many years including at Beijing 2008. He is now representing Sweden and aiming to qualify for Tokyo.
Turkey: Vedat Albayrak (judo) - Born in Kazakhstan as Vano Revazishvili. Apparently he is Georgian. He represented Greece in Rio 2016 under the name Roman Moustopoulos. Now he represents Turkey under the name Vedat Albayrak. I am very curious about him and his multiple different names and nationalities
USA: Stephania Haralabidis (water polo) - Born and raised in Greece to a Greek father and a Canadian mother. Won gold with Greece at the 2011 European Junior Championships, 2012 Youth World Championships, and 2014 European U19 Championships. Played for the Greek senior team until 2015. Now represents USA, where she has lived since she was 17 (2012). In Greece her name was Stefania Charalampidi, but I guess she changed the spelling to make it look more American. She also has a twin sister who played for Greece with her and it was rumored that she would switch to Canada (she probably wasn't good enough for USA) but she stopped playing completely. Stefania also still plays club water polo in Greece...
USA: Helen Maroulis (wrestling) - Born and raised in USA to a Greek father (I am not sure about the mother but I don't think she is Greek, I could be wrong though). Helen's grandparents still live on a very small Greek island called Kalamos and she visits quite often. She missed out on Team USA in 2012 and was allegedly planning on competing for Greece instead for Rio 2016, but apparently the Greek wrestling federation messed up or didn't care. After she won gold for USA in Rio 2016, they named the main square of Kalamos island after Helen.
USA: Alexander Massialas (fencing) - Born and raised in USA to a Greek father and a Taiwanese mother. Won two medals in Rio 2016 for USA.
USA: Nefeli Papadakis (judo) - Born and raised in USA to a Greek father and a Filipino mother.
Fully Greek origins and represent Greece but born and raised elsewhere:
Kosta Koufos (basketball) - Born and raised in USA. Both of his parents are Greeks.
Scarvelis siblings (athletics - women's hammer throw and men's shot put) - Born and raised in USA. Both of their parents are Greeks.
Elaina Tabb (athletics - middle distance) - Born and raised in USA. Both of her parents are Greeks. Her name used to be Elaina Balouris until she got married. I think there is a small chance that she can switch to Greece in the next months and compete in the Olympics which is why I included her.
Christos Tsakmakis (canoeing) - Born and raised in Germany to Greek parents. Represented Greece in Athens 2004, Beijing 2008, and London 2012. He is trying to make it to Tokyo 2020 now but I don't think it is very likely as he was out of competition for several years.
There are many others but I only included those who have chances for Tokyo, otherwise the list would be too long. If there are any more that I think of or discover, I will add them. I also did not include Greek-Cypriots unless they have roots from mainland Greece, otherwise 95% of Cypriot athletes would be on this list too