Login name and password are the required information to log in to Kintone.
It is recommended to establish a password policy so that users will not choose weak passwords during password configuration.

It must be 128 characters or less.
You cannot use a login name that fits one of the following:

• The login name is already used by another user.
• The login name starts or ends with a space.
• The login name contains a special character not allowed to use for a login name.

A login name cannot contain the following special characters:

• Yen sign, backslash
  \
• Bracket
  [ ]
• Double quotation mark
  "
• Colon
  :
• Semicolon
  ;
• Less than sign
  <
• Greater than sign
  >
• Comma
  ,
• Slash
  /

• Enter up to 64 characters.
• You can use alphanumeric characters and special characters.
• The user name is case-sensitive.
• Administrators can configure a certain level of password complexity rule (password policy).
  Users cannot configure passwords that do not satisfy the complexity requirements.

A password can contain the following special characters:

• Exclamation mark
  !
• Double quotation mark
  "
• Number sign
  #
• Dollar sign
  $
• Percent
  %
• Ampersand
  &
• Single quotation mark
  '
• Parenthesis
  ( )
• Asterisk
  *
• Plus sign
  +
• Comma
  ,
• Hyphen
  -
• Period
  .
• Slash
  /
• Colon
  :
• Semicolon
  ;
• Equal sign
  =
• Less than sign
  <
• Greater than sign
  >
• Question mark
  ?
• At sign
  @
• Bracket
  [ ]
• Caret
  ^
• Underscore
  _
• Backquote
  `
• Brace
  { }
• Pipe
  |
• Tilde
  ~
• Space

Configure it in "Security" in Kintone Users & System Administration.
For details, refer to the following page:
Specifying Password Complexity and Password Expiration