You need to encounter all 151 Pokemon on the native Sinnoh Dex in order to unlock the National Dex in Pokemon Brilliant Diamond and Pokemon Shining Pearl. It is not necessary to capture every Pokemon to do this, but you do need to at least see all Sinnoh region Pokemon which you can do through battles with NPCs and wild encounters. If you battle every trainer in every area on your journey to becoming Champion you will get most of the Pokedex entries. This How to Complete the Sinnoh Dex guide lists the Pokemon that you may miss on your journey, as they may reside in areas that aren't required for main story progression or aren't regularly featured in opponent trainer battles. If you want to be thorough and capture all the Pokemon you will need to trade Pokemon with others to get those version exclusives that don't appear in your game.
Torterra (#003)
Ace Trainer Sydney battle (Victory Road)
Empoleon (#009)
Veteran Edgar Battle battle (Victory Road)
Bidoof (#013)
Lass Natalie battle (Route 202)
Bibarel (#014)
Tuber Chelsea battle (Route 213)
Kricketot (#015)
Youngster Dallas battle (Route 203)
Kricketune (#016)
Bug Catcher Donald battle (Eterna Forest)
Shinx (#017)
Youngster Logan battle (Route 202)
Luxio (#018)
Rich Boy Jason battle (Route 212)
Abra (#020)
School Kids Christine and Harrison battle (Jubilife City)
Magikarp (#023)
Youngster Tyler battle (Route 204)
Budew (#025)
Lass Kaitlin battle (Route 203)
Graveler (#032)
Skier Bradley battle (Route 216)
Rampardos (#037)
Ace Trainer Omar battle (Victory Road)
Shieldon (#038)
Ruin Maniac Calvin battle (Route 215)
Machop (#040)
Youngster Sebastian battle (Route 203)
Machamp (#042)
Black Belt Miles battle (Victory Road)
Psyduck (#043)
Picnicker Diana battle (Oreburgh Gate)
Golduck (#044)
Swimmer Evan battle (Route 213)
Burmy (#045)
Bug Catcher Donald battle (Eterna Forest)
Only one Cloak needed
Wormadam (#046)
Beauty Devon battle (Route 214)
Only one Cloak needed
Mothim (#047)
Ace Trainer Ernest battle (Route 210)
Wurmple (#048)
Bug Catcher Brandon battle (Route 204)
Silcoon (#049)
Bug Catcher Jack battle (Eterna Forest)
Beautifly (#050)
Bug Catcher Jack battle (Eterna Forest)
Cascoon (#051)
Bug Catcher Phillip battle (Eterna Forest)
Combee (#053)
Aroma Lady Hannah battle (Route 208)
Pachirisu (#055)
Twins Liz and Liv battle (Route 204)
Cherrim (#059)
Ace Trainer Shannon battle (Route 221)
Picnicker Summer battle (Iron Island)
Shellos (#060)
Tuber Jared battle (Route 213)
Aipom (#063)
Rancher Marco battle (Route 210)
Drifloon (#65)
School Kid Mackenzie and Picnicker Cheyenne battles in Hearthome Gym (Hearthome City)
It is strongly recommended to fight these trainers in Fantina's Gym as these are the only trainer battles that feature Drifloon.
Buneary (#067)
Lady Melissa battle (Route 212)
Gastly (#069)
Psychic Abigail battle (Route 214)
Haunter (#070)
Psychic Mitchell battle (Route 214)
Misdreavus (#072)
Psychic Abigail battle (Route 214)
Glameow (#076)
Belle and Pa Ava and Matt battle (Route 210)
Goldeen (#078)
Fisherman Zachary battle (Route 205)
Seaking (#079)
Ace Trainer Olivia battle (Route 217)
Barboach (#080)
Fisherman Travis battle (Route 212)
Chingling (#082)
Psychic Valencia battle (Victory Road)
Chimecho (#083)
Psychic Valencia battle (Victory Road)
Stunky (#084)
Youngster Oliver battle (Lost Tower)
Bonsly (#092)
PKMN Breeder Albert battle (Route 209)
Mime Jr. (#094)
PKMN Breeder Jennifer battle (Route 209)
Happiny (#096)
PKMN Breeder Kahlil battle (Route 210)
Blissey (#098)
Ace Trainer Mariah battle (Victory Road)
Cleffa (#099)
PKMN Breeder Jennifer battle (Route 209)
Clefairy (#100)
PKMN Breeder Amber battle (Route 210)
Clefable (#101)
Ace Trainer Sydney battle (Victory Road)
Chatot (#102)
Gentleman Jeremy battle (Route 212)
Pichu (#103)
PKMN Breeder Albert battle (Route 209)
Pikachu (#104)
Cyclist Kayla battle (Route 206)
Hoothoot (#106)
Bird Keeper Alexandra battle (Route 211)
Noctowl (#107)
Bird Keeper Brianna battle (Route 210)
Gible (#109)
Dragon Tamer Clinton battle (Victory Road)
Gabite (#110)
Dragon Tamer Ondrej battle (Victory Road)
Unown (#114)
Catch inside Solaceon Ruins
Riolu (#115)
Veteran Grant battle (Oreburgh Gate B1F; requires Rock Smash, Surf)
Hatch Egg given by Riley on Iron Island
Wooper (#117)
Catch inside Great Marsh
Wingull (#119)
Catch at Route 213 while Surfing (Common)
Swimmer (Male) Colton battle (Route 223)
Pelipper (#120)
Swimmer (Male) Vincent battle (Route 220)
Hippopotas (#122)
Ace Trainer Dalton battle (Route 217)
Azurill (#124)
Swimmer (Female) Aubree battle (Route 223)
Marill (#125)
PKMN Ranger Allison battle (Route 212)
Azumarill (#126)
Swimmer (Female) Haley battle (Route 213)
Skorupi (#127)
Ninja Boy Joel battle (Route 210)
Croagunk (#129)
Black Belt Nathaniel battle (Route 215)
Carnivine (#131)
Ace Trainer Omar battle (Victory Road)
Remoraid (#132)
isherman Kenneth battle (Route 213)
Finneon (#134)
Fisherman Cory battle (Route 221)
Fish with Good Rod at Route 218
Lumineon (#135)
Swimmer (Female) Miranda battle (Route 223)
Tentacool (#136)
Swimmer ♂ Adrian battle (Route 220)
Catch at Route 213 while Surfing (Very Common)
Tentacruel (#137)
Swimmer (Male) Wesley battle (Route 223)
Catch at Route 213 while Surfing (Very Rare)
Feebas (#138)
Sailor Luther battle (Route 222)
Mantyke (#140)
Swimmer (Male) Adrian battle (Route 220)
Mantine (#141)
Skier Bjorn battle (Route 217)
Snover (#142)
Skier Shawn battle (Route 217)
Uxie (#146)
Cave in Lake Acuity, after beating Cyrus at Spear Pillar
Mesprit (#147)
Cave in Lake Verity, after beating Cyrus at Spear Pillar
Azelf (#148)
Cave in Lake Valor, after beating Cyrus at Spear Pillar
Dialga (#149)
Spear Pillar (Brilliant Diamond)
Talk to Cynthia's grandmother in Celestic Town, after beating Cyrus at Spear Pillar (Shining Pearl)
Palkia (#150)
Spear Pillar (Shining Pearl)
Talk to Cynthia's grandmother in Celestic Town, after beating Cyrus at Spear Pillar (Brilliant Diamond)
Take a look at our Ramanas Park Guide to find out how to unlock the Legendary Pokemon that can be found there.