Follow these simple steps.

Beat "Competition Mode" as Strength. This will unlock Father Strength!

After that while on character selection screen, select Strength then press the "Drop" button (default L1, L2, R1 and R2). You will see a picture of Strength's Father, press X to play as him.

Complete "Adventure" mode with young Strength. This will make father Strength playble in other modes.
Once unlocked, highlight young Strength on the character select screen and press L1 or R1 to make father Strength appear. Press circle or triangle to choose father Strength while his portrait is shown.

During the game, when the card begins to spin, but before it stops spinning, press start as fast as you can. Each time you press start, the difficulty will increase by one, to a maximum of seven.

There are six hidden characters that can be unlocked to be used in challenge mode and against another player. What you need to do is highlight a certain character and press the C button three times within a second.

It isn't as bad as you think though, since you can choose which second in the countdown to press the C button three times.

20 - Judgment
19 - Sun
17 - Star
15 - Devil
13 - Death
11 - Justice
8 - Strength
7 - Chariot
6 - Lovers
5 - Hierophant
4 - Emperor
3 - Empress
2 - High Priestess
1 - Magician
0 - Fool

If done correctly you will see the hidden characters appear about a second later. The characters unlocked this way are: Moon, Hanged Man, Hermit, Temperance, Tower, and Fortune. You will also gain 10 seconds to the countdown, which can go up to the max of 20.

To unlock Father Strength in the same modes as the others, all you need to do is hold the C button and select Strength with the A and B button at the same time.

Hit B on any character to get their alternate colors.

Highlight Strength and while holding the C button, press either the A button or the B button to play as Father Strentgh. The button you pressed determines his outfit.

There are six hidden characters that can be unlocked to be used in challenge mode and against another player. What you need to do is highlight a certain character and press the C button three times within a second.

It isn't as bad as you think though, since you can choose which second in the countdown to press the C button three times.

20 - Judgment
19 - Sun
17 - Star
15 - Devil
13 - Death
11 - Justice
8 - Strength
7 - Chariot
6 - Lovers
5 - Hierophant
4 - Emperor
3 - Empress
2 - High Priestess
1 - Magician
0 - Fool

If done correctly you will see the hidden characters appear about a second later. The characters unlocked this way are: Moon, Hanged Man, Hermit, Temperance, Tower, and Fortune. You will also gain 10 seconds to the countdown, which can go up to the max of 20.