Successfully complete the indicated task to unlock the corresponding character. Press Up at the character selection screen to display the bonus characters.

Alia: At South Alps hole 14, talk to Alia with the magic broom equipped.
Alice: Defeat her in Beginner VS. mode.
Britz: At El Andes hole 17, use Frau Ada to lift up a rock in the bare ground on the left.
Chad: Defeat him in Platinum VS. mode.
CJ: At Olive Coast hole 4, talk to him with a beautiful female (for example, Patricia).
Frau Ada: Defeat her in Pro VS. mode.
Gloria: At Fortress Yard hole 16, talk to her with Chad dressed as a girl.
Holly: At Dino Park hole 6, have Malachy play hide and seek with the woman dressed as a tree off the back of the green.
Julie: At Autumn Pagoda Golf Course hole 17, talk to Julie using Mifune.
Lauryn: Defeat her in Bronze VS. mode.
Lee: Defeat him in Gold VS. mode.
Logan: At Golden Desert hole 13, have Big Impact clubs.
Malachy: Defeat him in Amateur VS. mode.
Mifune: Defeat her in Silver VS. mode.
Mika: Chip in a shot at Nakagawa hole 14, and talk to Mika.
Mizuki: Unlocked randomly as a character while you are playing as Mizuho.
Montcalm: Defeat him in Seeded-Pro VS. mode.
Patricia: At Gameland Island Resort hole 1, talk to Patricia while wearing a swimsuit.
Rio: Bogey hole 5 at Central Golf Course, then talk to Rio in the right most cherry blossom tree on hole 6.
Roger: At El Andes hole 17, lift the rock in the bare ground to the left with Frau Ada.
Shu: At Michinoko hole 15, talk to Shy with a female character.

Search the indicated hole to find the corresponding hidden item:

Aikidu Sensei: Autumn Pagoda, hole 13
Baquette Club Intermediate: Central Golf Square, hole 2
Baseball Helmet: Nagawaka, hole 3
Beginners Club Intermediate: Olive Coast, hole 10
Big Air Club Expert: El Andes, hole 18 as a male character
Big Paper Fan: South Alps, hole 3
Big Serve Club Intermediate: Gamelan Island Resort, hole 4
Black Afro: Gamelan Island Resort, hole 6
Bottle Club Expert: Nagawaka, hole 13
Busby Hat: Fortress Yard, hole 15
Caveman Outfit: El Andes, hole 12
CH Paddle Club Expert: Fortress Yard, hole 17
Clown Car: Central Golf Square, hole 17
Cricket Helmet: Royal And Links, hole 17
Flamenco Dress: Fortress Yard, hole 7
Flight Steward: Dino Park, hole 14
Flower Kimono: Nagawaka, hole 16
Gloria's Outfit: Central Golf Square, hole 12 as Gloria
Gnome Beard: Golden Desert, hole 2
Golfing Trousers: South Alps, hole 7
Infinity Club Intermediate: Royal And Links, hole 7
Iron Mace: Autumn Pagoda, hole 16 as a male character
Karate Uniform: Michinoku Pass, hole 17
Kung Fu Costume: El Andes, hole 10
Leisure Suit: Olive Coast, hole 6
Maid's Headpiece: Olive Coast, hole 18
Mandarin Gown: Gamelan Island Resort, hole 16
Monkey's Costume: Golden Desert, hole 14
Navy Sweater: Royal And Links, hole 3
Ninja Outfit: Autumn Pagoda, hole 5
Pin hole Club Intermediate: Golden Desert, hole 12
Royal Hanbok: Dino Park, hole 17
Rugby Uniform: Central Golf Square, hole 14
Samurai Kimono: Autumn Pagoda, hole 8
Soccer Uniform 2: Royal And Links, hole 5
Stable Boy: Michinoku Pass, hole 7
Tiger Head: Dino Park, hole 4, on the left about halfway to the green
Turbo Spin Club Beginner: South Alps, hole 17

Successfully complete the indicated task to unlock the corresponding item:

Baby On The Go: Sink a greater than 60 foot putt.
Biker suit: Get a Hole In One.
Chief Seven Iron outfit: Score over 400 points in the mini-game.
Comedy suit: Score over 600 points in the mini-game.
IV Drip: Get a Rising shot.
Knight Armor: Score over 700 points in the mini-game.
Priestess Kimono: Get a Double Eagle.
Knight helmet: Hole out with the Homing Ball animation.
Pink Outfit: Score over 300 points in the mini-game.
Soccer Uniform 1: Score over 500 points in the mini-game.
Wedding Veil: Get a Flag Ball shot.
Yukata: Get a Spiral shot.

Acquire all cards to reach the "Black" rank.

Select Challenge mode, and play a VS. match. Press Start when the match begins, and select the "Give Up" option. Repeat this three times to intentionally lose the match and increase the loyalty with that character.

Play Challenge mode, and reach the Seeded Pro level. Look at the leaderboard after each hole, and look at your opponents' names. Some of the names listed are references to characters from Futurama, which are as follows:

"Ben B." and "Rodriguez" are references to Bender.
"Conrad" is a reference to accountant Hermes Conrad.
"Fry" is a reference to Philip J. Fry.
"Leeland" is a reference to Leela.
"Zoe Berg" is a reference to Dr. Zoidberg.